This series explains the origin of Sonic and his siblings and how they banded together to reunite with their mother. Queen Aleena tells the story of how Dr. Robotnik took over planet Mobius and how she had to give up her three children–Sonic, Sonia, and Manic–to fulfill a prophecy that one day the siblings will be able to rise against Robotnik's rule and restore peace to the planet. This episode also shows where Sonic, Manic, and Sonia end up living. Sonia is left on Lady Windimire's doorstep. Sonic lived with two Mobians who ended up getting roboticized, forcing Sonic to live with Uncle Chuck. But when Manic is left on a doorstep, he is kidnapped by thieves, who raise him in their ways. The Hedgehogs find each other and see their guardians roboticized, and discover more about their mother.
The Hedgehog siblings begin to learn the important lessons of what it takes to be a Freedom Fighter. The Oracle provides teachers to guide Sonic, Manic and Sonia to develop the necessary skills to recognize the power that comes from within. The Hedgehogs also discover the hidden magical force of the power medallions as they grow to understand that teamwork will serve them best in averting the dangers that await them.
Manic, Sonic and Sonia must end their training early when Dr. Robotnik threatens the Oracle and takes a member of the resistance hostage. During the rescue mission, the Hedgehogs fall into trouble and begin to place blame upon each other for everything that has gone awry. Queen Aleena then intervenes with an important message for her children that they must learn to work together and derive strength from their willingness to help others.
Argus, a captain of the Queen's guard, is captured by Sleet and Dingo before he can reach the secret rendezvous where he was to meet Queen Aleena. While listening to a scanned radio message, Sonic, Manic and Sonia overhear that a queen's guard has been captured and roboticized. The hedgehogs then begin their pursuit to recapture Captain Argus from Dr. Robotnik and get the information revealing the secret meeting place where they may be reunited with their beloved mother.
Sonic, Sonia and Manic return to their birthplace, Mobodoon, a beautiful, festive town. The homecoming is interrupted when Sleet and Dingo arrive to collect back taxes by order of Dr. Robotnik. Because Mobodoon has no form of currency to pay this bogus debt, Sleet and Dingo decide to raid the village and steal its most precious monuments, including the powerstone, which is the center of Mobodoon's vitality. It is up to the Hedgehogs to get the powerstone back and restore their former home to its previous blissful state.
Sonia travels to east Mobius to visit her friend Mindy at the LaTour Estate. After being given a watch by her friend Mindy, Sonia discovers that these popular timepieces are really monitoring devices for the Robotnik Intelligence Agency and are mass produced by Mindy's father Mr. LaTour. Sonia, Sonic and Manic confront Mindy and LaTour with this discovery. Realizing the error of his ways, Mr. LaTour ends production and decides to flee with his daughter. The hedgehogs skillfully put the spy devices to their own use to help the LaTour's escape.
Sleet and Dingo plan to collect the taxes owed to Robotnik from the Duke of Velcronia but keep it for themselves. To avoid the wrath of Robotnik, Sleet and Dingo infiltrate the Underground to corral Manic and friends to the commit the crime and take the blame. The scheme is moving along well for the evil duo until they are caught on tape by Max, Manic's friend, who stops the devious Sleet and Dingo enabling the Duke keep his money.
The sinister Robotnik blackmails Cyrus, an old friend of Sonic's, by holding his father hostage. Robotnik makes Cyrus gain the Hedgehogs trust and then has Cyrus convince them to save his father. Cyrus reluctantly informs Robotnik of the rescue plan so Robotnik can devise a trap for the Hedgehogs. However, once Cyrus sees the Hedgehogs helping the kids of Mobius, he realizes assisting in the capture of the Hedgehogs is a bad idea. Instead, Cyrus helps the Hedgehogs and in the process they destroy another of Robotnik's swat-bot factories.
On a trip to Port Mobius, Sonic, Manic and Sonia are told that ships are sinking due to the Mobian sea-beast. The legendary story becomes fishy when the Hedgehogs meet Captain Squeegee who informs them of a scam in which townspeople must rely upon Robotnik for protection from the sea monster, when in reality it is Robotnik who is sinking ships, not a monster. Sonic must overcome his fear of water and the monster to find the truth and thereby ending the fears of the sea side town.
Queen Aleena's royal journals are rumored to be buried under a palace in the city of Tashistan. As Sonic and his siblings scour the city in search of the scrolls, Sonia gets into trouble when she is accidentally knocked in the head during a confrontation, causing her to lose her memory. Sonia gets into more trouble when she inadvertently sells her power medallion to a local merchant. A young boy from the village makes an attempt to keep Sonia safe, but the two are captured by Sleet and Dingo. Sonic and Manic must then work together to retrieve the power medallion, locate the journals and find the one thing that is most precious to them, Sonia.
Sonic, Sonia, and Manic are looking for the Royalcy of Mobius in an abandoned old factory. They are ambushed by Dingo, Sleet, and a bunch of Swatbots. The three hedgehogs destroy a number of Swatbots and escape. They get in their van and Sonic drives them through a wasteland to the hilly Valley Resort. They are greeted by Stripes, who welcomes them and puts them up in a nice room. Stripes seems to be interested in Sonia. Sonic is suspicious of Stripes' loyalty.
Stripes is in contact with Robotnik. He tells him that he has two visitors instead of three. Robotnik says that if they are members of the resistance he must roboticize them.
The next day Sonic and Manic go exploring around the village. Stripes comes to their room and courts Sonia. Manic mentions being part of the resistance to a citizen and the whole town turns against him. Sonic saves Manic and they figure out this place is strange because the people side with Robotnik.
While on a date with Sonia, Stripes explains that the only way to survive is to make a deal with Robotnik. Sonia disagrees and makes Stripes stomp away angry. Sonia sings a pop song. Manic and Sonic find Sonia and they suspect that Stripes works for Robotnik.
Later that night, Sonic spies on Stripes as he calls Robotnik. Stripes says Sonic and Manic are there. Robotnik asks about Sonia. Stripes lies and says he has not seen her. Robotnik hangs up on Stripes and orders Sleet and Dingo to go destroy the valley, insisting he doesn't need Stripes' help anymore.
Sonic tells his siblings that Stripes ratted them out to Robotnik. Sonia gets angry and says she doesn't believe him. The Hedgehogs confront Stripes and he tells them the truth. He explains that he is only in cahoots with Robotnik to protect his valley and that the resistance is useless. He says he told Robotnik about Sonic and Manic, but not Sonia. Sonia says she would never abandon her brothers or the resistance.
Sleet and Dingo show up with Swatbots and prepare to roboticize everyone in the valley. Machines start destroying all the trees. The Hedgehogs get away, but Sonia doesn't want to run away, she wants to help Stripes. The Hedgehogs return and rescue Stripes from the roboticizer. All the citizens come out of their homes and attack the Swatbots. Sonic destroys the robots and they throw Sleet and Dingo out.
The next day, the hedgehogs are leaving. Stripes and his people tell them they have joined the resistance and invites Sonia to fight by his side. Sonia insists on staying with her brothers and they depart hoping to meet Stripes again some day.
Sonia is invited to a debutante ball, but before she can be officially announced to society, she and her brothers, Manic and Sonic have an important mission. Robotnik has constructed an evil weapon that will capture and robotisize all members of the resistance all over the world. The Underground must hijack this weapon and insert a deactivating device before the brain of the vehicle is installed. Sonia daydreams about the party, becomes distracted and is caught while executing part of the mission. Sonic aids Sonia to help her focus on their mission and still make an appearance at the ball.
Robotnik organizes a track meet to lure Sonic into a trap. Robotnik is sure that Sonic will not sit out on a challenge of quickness. Sonia and Manic are also aware of Sonic's competitive nature and advise Sonic not to enter the event due to the risk that he may be caught. When Sonia and Manic cannot find Sonic, they head to the race to rescue him. Surprisingly, Sonic does not enter the race, but finds himself involved in a more challenging test of speed - the race to save his brother and sister.
Manic is exploring the sewers and entering numbers into a calculator, when suddenly, what appears to be Queen Aleena passes by right in front of him. He calls out and runs after her, but she doesn't stop. She just drops a letter on the ground, which Manic picks up and soon takes back to his siblings. Shortly after, it is revealed that she was actually Dingo transformed into Aleena - he rendezvouses with Sleet and confirms that Manic has taken the letter, setting their plan in action.
The letter is an invitation to Worst Castle, addressed to Sonia from her fictitious cousin Albert. Sonia is excited about having her own castle, but Sonic is reluctant to attend, so Sonia heads off with Manic while Sonic stays behind. He feels hungry for chili dogs and comes across a chili dog stand run by Dingo, who he initially doesn't recognize until he speaks his name. Sleet and Dingo then try to shoot Sonic down in a gunship, but he easily defeats them by jumping on top of the ship and distracting them so that they crash into a wall.
Sonia is enchanted by the sight of Worst Castle, but Manic is a little bit creeped out. As they enter, a flash of lightning reveals a scary-looking vampire bat called Bellok, the castle caretaker, and his owl Hooter. Bellok welcomes them to the castle and shows them their rooms, but they are infested with creatures and Manic becomes frightened. Sonia finds him ridiculous and insists that they at least give Worst Castle a chance. Just then, Sonia spots her mother for a split second, but Manic finds no evidence at all and suggests that they go to bed. That night, while Sonia is trying on a dress in the mirror, she sees something unnatural in its reflection - this is no ordinary mirror! Manic is disturbed by a creature lurking in the corridor, but it's only Sonic; he wanted to show off his new anti-gravity shoes that allow him to walk on walls and ceilings. Sonia arrives and tells Sonic that she saw their mother, and Sonic storms out to look for her. He ends up having an awkward encounter with Bellok and almost gets crushed by a falling chandelier, making him suspicious. Meanwhile, Sonia has been looking in the mirror and has seen Queen Aleena on the other side!
Sonic follows Manic down a trapdoor into a dungeon, which they escape by tearing the exit down with their instruments. Upon returning to Sonia's room, they find no answer from her but they are unaware that she went into the mirror. Sonic spots Bellok nearby and runs after him and stops him to ask him what's going on. Bellok refuses to explain it to him at first, but after being tortured by Sonic's loud and heavy guitar playing, he confesses that Sleet threatened to eat Hooter if he didn't aid his scheme to destroy the Underground. He begs Sonic to protect him and Hooter and warns that Sleet will arrive in less than an hour! Manic has meanwhile picked the lock on Sonia's door and discovered that she is inside the mirror. Sonic steps inside and finds Sonia, who was looking for Aleena but ended up getting lost. They find that running at high speeds is futile (and painful) so they walk slowly and eventually get out of the mirror. Outside, Aleena touches a sleeping Manic's forehead and tells him that the mirror is the way out, and disappears before he wakes up and Sonic and Sonia return from the mirror. Manic felt Aleena's touch and thought he was dreaming. Suddenly, Bellock rushes up to announce that Sleet has arrived with a group of Swatbots ready to attack! The hedgehogs perform an upbeat song and, with the help of Bellok, counter the Swatbots by utilizing the castle's many trapdoors, chandeliers and revolving fireplaces.
Enraged, Sleet calls all units and has Worst Castle surrounded by elite enemy ships. The four heroes have no hope of defeating them. All at once, Manic remembers Aleena's words from his "dream" and tells the others to escape through the mirror. Sonia is worried that they could become eternally trapped if the mirror is broken, but she and Sonic realize there is no other way and have to trust Manic's "dream". The hedgehogs and Bellok head into the mirror's dimension, just in time to escape an oncoming missile which shatters the mirror. While inside, the heroes are engulfed in a golden whirlpool which transports them to a clifftop overlooking Worst Castle, which becomes decimated by missiles. Manic recalls his "dream" and how Aleena's touch felt so vivid, and Bellok reassures him it really happened. Bellok turns around and nods to Queen Aleena, who is standing nearby with a tear in her eye.
Doctor Robotnik is hosting a party for the aristocrats of Robotropolis, where he is about to make an important announcement that could mean trouble for the Resistance. Accordingly, Sonic, Manic and Sonia are conducting espionage by dressing as waiters and offering hors d'oeuvres to the party guests. After a while, Robotnik appears and explains the reason why he has invited (or rather ordered) the guests to this party: he's just discovered that the legendary lost city of Mobupinchu really exists! Robotnik has brought with him a pair of old-looking objects, which he claims are priceless artifacts that he recovered from Mobupinchu. He then begins an auction to sell the artifacts as a fundraiser for building more Swatbots, setting the opening bid at a massive 100,000 Mobium. Sonic, Manic and Sonia are disgusted that Robotnik is looting Mobupinchu's treasure just so he can generate cash for his own cause. Sonic then trashes the place and sends the guests running. Robotnik commands some Swatbots to stop them, but the hedgehogs seize both of the artifacts and escape the building by parachute.
At his base, Robotnik orders Sleet and Dingo to destroy the hedgehogs and bring back the artifacts they stole, while Robotnik himself goes to Mobupinchu to supervise an archaeological operation to find the true city. Back in Robotropolis, the hedgehogs realize that they must locate Mobupinchu in order to foil Robotnik's raiding schemes. Sonia decides to visit the Professor at Mobius University to see if he can glean anything from the missile-shaped artifact she grabbed. Sonic joins her, but Manic waits behind, intent on trading the orb-shaped artifact he concealed in his fanny pack for money to fund the Resistance.
At Mobius University, the Professor uses the Gamma Spectrum Analyzer to examine Sonia's artifact. To their surprise, the artifact is not from Mobupinchu; it's a bomb from a war that took place only 500 years ago! This is clearly an answer that Robotnik wouldn't want to hear, so his experts deliberately gave him misinformation. Meanwhile, Manic convenes with a shady salesman called Vince, who buys his orb for 2,000 Mobiums. Wondering what's inside, Vince pushes a button on the orb and discovers that this artifact is also a bomb, and it will explode in 12 minutes!
Sonic and Sonia meet up with Manic to tell him the news about the artifacts, and Manic ruefully admits that he had taken Robotnik's orb and sold it to raise money for the Resistance. The hedgehogs now have to retrieve the orb before it detonates. Manic leads his siblings to Vince's cave and tries to purchase the orb back from him, but Vince has already sold it to an art lover. Vince selfishly declines to tell them who bought the orb, and in anger, Sonic spins him around very fast. Vince hurtles into a wall and falls down. Feeling threatened, he reveals that he had sold the orb to Bartleby.
Vince was expecting to be paid for this information and becomes annoyed when the hedgehogs urgently dash off to Bartleby's mansion to get the orb back. He gets his retribution when he conspires with Sleet and Dingo and tells them the whereabouts of the orb in exchange for cash. Unfortunately for Vince, Sleet goes back on his promise and has him roboticized. At Bartleby's mansion, the hedgehogs are trying to convince Bartleby to hand over the orb when Sleet and Dingo burst in. They kidnap Bartleby and the orb and depart for Mobupinchu.
The hedgehogs follow Sleet and Dingo's trail through the mountains. At one point, Sonic finds a shortcut and whisks Manic and Sonia up a mountain, making the latter dizzy and nauseous. Manic then points out that they have entered the valley of Mobupinchu! Sleet and Dingo bring Bartleby before Robotnik, who is about to punish him for getting the orb cheap, when Sonic intervenes and snatches the orb. Everyone scrambles around to get their hands on it, and it ends up in Bartleby's lap. Suddenly, the orb makes a loud beeping noise - the bomb is about to explode! The hedgehogs get their instruments out and play her song as the bomb detonates. The explosion triggers several other bombs buried in the area, wrecking Robotnik's digging operation and forcing Robotnik, Sleet, Dingo and Bartleby to flee in panic.
After the performance, the hedgehogs find a tunnel and walk through it. And what should lie at the end of the tunnel but the real city of Mobupinchu! The amazed hedgehogs run into a treasury containing mounds of gold pieces and gemstones. Manic picks up a precious stone to keep for himself, but Sonic reproaches him, pointing out a "QA" symbol on the floor. These are the initials of Queen Aleena, their mother, indicating that she is guarding this treasure. The treasure belongs to all the people of Mobius, and it's the Resistance's job to keep it safe. Manic wishes their mother could have witnessed their victory over Robotnik, but Sonic feels certain she already knows about it. And indeed she does - she has been watching them from a cliff top all this time.
Manic surfs the air thermals in the sky over a large lake on a small red surfboard. He is attacked by a giant Stealthbot that rises out of the water and fires at him. Manic dodges and weaves in an attempt to escape. Sonic runs up and catches the robot's leg with a lasso, spinning it around and then smashing it to pieces against the rocks on the lake's shore. However, the robot's radio receiver is still active, and Sonic hears a message from Sleet saying that Queen Aleena has been spotted in Quadrant 10. In order to allay suspicion, Sonic imitates the Stealthbot's robotic voice and confirms his intention to search Quadrant 10, then disables the radio. Sonic then finds Manic (whose surfboard crashed into a cliff) and demands to know why he was out all alone, as it is dangerous and they should always go out in pairs. Manic just says he was surfing thermals, and Sonic says that he likes to surf thermals too (and is very good at it). They return to the van to tell Sonia about Sleet's message.
The van soon arrives in Quadrant 10, a bleak wasteland. Sonia wonders why Aleena would come to such a place. The van is being driven by Manic, who has a habit of hitting every pothole he passes, leading his siblings to complain. Pretty soon, the van needs a wheel change, and while they're stopped they are approached by a tall, leather-wearing lizard in another car. Although they are initially suspicious of him, he reveals through a secret codephrase that he is actually a local Freedom Fighter named Rudy.
Rudy hasn't seen Queen Aleena, but he says that's a good thing because he's been having problems. The previous night, while returning to his base, he saw a swarm of bright orange bugs, like bottle flies, leaving the village. Immediately afterwards, he was attacked by bis own friends, other Freedom Fighters, who tried to take him to Robotnik. He suspects the bugs were involved, and Sonic Underground agree to help him investigate.
Sonia is disgusted by the thought of bugs. She suggests that Sonic investigate the village, after which they will all attempt to go and catch one of the bugs. Manic wants to go with Sonic, but Sonic says there's no time and tells Manic to stay behind. However, once Sonic is gone, Manic decides that if Sonic can go out alone then so can he, and he flies off on his thermal surfboard to look for a bug himself.
Sonic finds the village burned down and no sign of the Freedom Fighters. However, he sees one bug, which he catches in a glass jar, joking "You're a real bug and I'm a 'swat'-bot." Elsewhere, Manic also encounters a bug, which stings him on the nose. Suddenly his eyes dilate and in a monotone voice he repeats "Must report... must report to Robotnik..." before flying away on his board.
Sonic returns to Rudy and Sonia, but there's no sign of Manic. Sonic gives Rudy the bad news about his village, then shows them the bug. They discover that it is definitely Robotnik's, as it is actually made of metal. Inside, they contain a mind control serum that makes people obey Robotnik if stung.
In Robotropolis, Sleet reports to Robotnik on the testing of their new Flybots. He explains that a full does lasts an hour, which is long enough to compel a person to come to Robotropolis to be roboticized. Sleet also says that they burned down the Freedom Fighter village on Robotnik's orders, but when Robotnik asks for an update on the search for Aleena, Sleet is forced to admit that they haven't heard from the Stealthbot, and Robotnik orders him to find it. Manic arrives at Robotropolis, completely under Robotnik's control. Robotnik develops a plan to use Manic to capture the other Freedom Fighters. They put a modified, remote-controlled Flybot into his pouch, which will administer a micro-dose of serum when activated by radio. They'll take Manic back to the desert before the effects of the serum wear off, and use him to deliver the other hedgehogs to Robotnik.
Manic returns to the group with no memory of where he's been, saying only that he chased a Flybot and it got away. The others say that they plan to use the cannons mounted on the roof of the van to shoot the bugs. That night, Manic is stung and reports in to Robotnik, revealing their location. A short while later, Sonic wakes up (Manic is back sleeping) and sees Robotnik's forces heading for them, so he drives the tank away before they arrive. Rudy says to hide the tank in a cave he knows. On the way, Manic is stung again and reports in to Robotnik, but he isn't able to tell Robotnik where the hedgehogs are going before Sonia spots him and takes the radio. They realize he has been stung and find the bug in his pouch, which Sonic destroys with his guitar medallion.
Sonic and Rudy develop a plan to destroy the bugs. They intend to use the van as bait to lead the bugs to a place Rudy knows where they can be destroyed. While Rudy drives, the hedgehog siblings sing a pop song in the back of the van, which attracts the bugs' attention. They reach an area in the middle of an electrical storm. Sonic leaves the van and flies around on Manic's thermal surfboard, luring a group of bugs into the storm where they are destroyed by a lightning strike (Sonic is also struck four times). The other bugs are blasted by the other Freedom Fighters with the van's cannons.
Rudy and the others return to the village, where everything has been burned. However, some Freedom Fighters begin to emerge from the wreckage, including a female lizard named Sheila. She explains that Swatbots burned the village, but some of them escaped. Rudy is glad to be reunited with them.
In the Hanging Gardens, a place Queen Aleena built for the royal siblings. It then cuts to Robotnik cutting down the trees until finding the fabled Velocitree. At his mobile lab, he then proceeds to processing the tree, commenting it is the fastest growing thing on Mobius; with its essence, anyone can have sonic speed. The result is a bright yellow liquid, now called "Sonic Tonic". Dingo is a test subject for the mix. Sleet takes it after Dingo speeds away. Remarking on how the concoction may be dangerous, Sonic forbids his siblings from using it, leading them to remark behind his back that he needs to be "put in his place" and stop thinking that just because he's the fastest he's the leader of their team.
After putting a system in place, Sonic hangs around Robotropolis and is captured. However, after he is put in prison, Sleet and Dingo's feet grow to gigantic size, which leaves them unable to stop Sonic from escaping with the Sonic Tonic. When they have a blowout, Sonic leaves Sonia and Manic to fix the tire, which makes them use the Sonic Tonic. The same thing happens to them, but Sonic fixes it and the Hanging Gardens.
The Hedgehogs converge on the Floating Island, in their latest attempt to find their Mother. Sleet and Dingo also have an interest in this island that involves the Chaos Emerald, which is the island's life-force. But, before either party can achieve their goals, they have to get through Knuckles, the floating islands stern guardian. Knuckles is even skeptical of Sonic's motives, although he can see right through Sleet and Dingo. Once the Hedgehogs convince Knuckles they are friendly and mean no harm to his island, they help Knuckles in protecting his exotic home and the emerald.
- Sonic Underground
- Keiko Toda as Sonic the Hedgehog, Sonia the Hedgehog & Amen Rabi (Sonic's ancestor)
- Megumi Ogata as Manic the Hedgehog
- ??? as Sonic's adopted Mother
- ??? as Sonic adopted Father
- ??? as Sir Charles "Uncle Chuck" Hedgehog
- ??? as Ferrell
- ??? as Lady Windermere
- ??? as Queen Aleena
- ??? as Knuckles the Echidna
- ??? as Bartleby
- ??? as Oracle of Delphius
- ??? as Cyrus (Sonic's childhood friend)
- ??? as Mindy LaTour (Sonia's childhood friend)
- ??? as Mr. LaTour (Mindy's Father)
- ??? as Knuckles' Great Grandpa
- Koichi Hashimoto as Dr. Robotnik
- ??? as Sleet
- Tomohisa Aso as Dingo
- ??? as Swat Bots
- Jaleel White as Sonic, Sonia, Manic & Amen Rabi
- ??? as Sonic's adopted Mother
- ??? as Sonic adopted Father
- Maurice LaMarche as Ferrell, Sir Charles "Uncle Chuck" Hedgehog, Sleet, the Oracle of Delphius, Knuckles' Great Grandpa & Swat Bots
- ??? as Lady Windermere
- Gail Webster as Queen Aleena
- Brian Drummond as Knuckles the Echidna
- Phil Hayes as Bartleby
- Ian James Corlett as Cyrus (Sonic's childhood friend)
- Louise Vallance as Mindy LaTour (Sonia's childhood friend)
- ??? as Mr. LaTour (Mindy's Father)
- Garry Chalk as Dr. Robotnik
- Peter Wilds as Dingo
- ??? as Swat Bots
- Though the Freedom Fighters were included, many of the characters in the Freedom Fighter group that were in Sonic the Hedgehog are completely left out (including Tails). Another difference with the Freedom Fighter group is that Freedom Fighters do not remain in one Knothole-like refuge but instead travel around Mobius to battle Robotnik's forces on a global scale; and that Robotnik has left most of the Mobian people de-roboticized, leaving multiple cities, a poor underclass and an aristocracy for the heroes to interact with. Another notable difference is Robotnik's choice of law enforcement in the two continuities. While both versions featured SWATbots as Robotnik's grunt troops, their designs differed by a mile and more. The SatAM SWATbots also all looked identical, while the Underground SWATbots had two different head designs.
- This is not a sequel or spin-off of any of the other earlier "Sonic" programs, but a new series altogether.
- This is the only DiC Sonic the Hedgehog cartoon to have Jaleel White being the voice of more than one character.
- The last DiC Sonic the Hedgedog cartoon to feature Jaleel White as the voice of Sonic.
- Sonia is the only triplet to have a neck while Manic and Sonic don't have one.
- Sonic's mohawk was redesigned to have three rows, along with his teeth being shown more and his eyelids are more visible.
- According to the oracle's prophecy, the triplets someday will meet with their mother, and will join forces to defeat Robotnik and return everything to normal. But due to the series getting canceled before the original number of episodes were made, this never happened in any episode.
- Sonic, Sonia, and Manic often refer to Robotnik by the derogatory nickname "Robuttnik" or "Buttnik."
- This is the first Sonic television series to feature Knuckles and only one to not feature Tails.
- It is also the only feature show to a relative of Knuckles: his great-grandfather Athair (originally featured in the Archie comics), who was thought to have been voiced by Sean Connery, while it was in fact voice actor Maurice LaMarche.
- In the episode "Sleepers", Manic's voice is a little deeper than normal, whereas in the episode "Wedding Bell Blues," Sonic's voice was more high-pitched than normal. *In "To Catch a Queen," Manic's voice sounded more like Sonic's as he commented on parts he was stealing for an invention. Sonic's pronunciation of "Sonia" also differed throughout the series. This was probably an oversight by Jaleel White, who voiced all three siblings.
- The Archie Sonic comics consider Sonic Underground as an alternate universe version of Sonic, and has made one appearance in the series, in Sonic Super Special #10.
- There are only a few references to the games, including a circular floating fortress that resembles the Death Egg, the Grabber robots from Sonic 2, a large Freedom Fighter-eating spaceship called Predator which closely resembles the Egg Carrier, and Floating Island (Now known as Angel Island) which first appeared in Sonic the Hedgehog 3 along with Knuckles.
- In the first episode "Beginnings," just at the beginning, there is a statue that strongly resembles King Acorn from the SatAM series.
- Some fans have commented that the sanctuary at the end of episode 20, "Three Hedgehogs and a Baby," resembles Knothole Village from Sonic the Hedgehog (TV series).
- The SWATbots in "Who Do You Think You Are" have a different voice.
- Brian Drummond (Knuckles), Samuel Vincent (Sonic's singing voice), Ian James Corlett, and Terry Klassen eventually appeared in the 2000s series Baby Looney Tunes as Floyd (Season 2 only), Baby Bugs, Baby Daffy, Baby Tweety, Baby Taz and Baby Sylvester respectively.
- In the episode Wedding Bell Blues, when the trio are carpet cleaners, Manic and Sonic look like the Mario Brothers.
- In the episode Mobodoon, Manic states that, "when the moon is full on the second even day of the third month, BAM, it shows up just like clockwork. Today's the day, and I just know that when Mobodoon shows up, so will Mom!" In that particular episode, the hedgehogs had stated that it was their birthday. So, their birthday is, in fact, March 4.
- Sonic cried once through the whole series ("Beginnings"), Manic cried four times ("Mobodoon," "Three Hedgehogs and a Baby," "Six is a Crowd," and "The A.R.T. of Destruction"), and Sonia cried the most out of all (nine times).
- Sonic's last words in the series were "Game over, Buttnik!"
- There are several errors that always occur, such as when Medallions are missing, Manic has his earrings on his right ear, Manic's fanny-pack is missing, and gloves or arms are in a different color. There have also been perspective issues.
- In the episode, "The Big Melt," when Manic comes out of the water and coughs, it's in Sonic's voice, and in "Beginnings" and "Haircraft in Space," you can see Sonic's mouth move to Sonia's voice, and vice versa.
- In the episode "Beginnings," when the photo of the three is shown, they are not babies, but toddlers. Aleena separated them when they were babies.
- The medallion strings are in different colors rather than what many people think to be green, blue, and pink. Sonic has a red string, Sonia has a yellow string, and Manic has a blue string.
- In the episode "Underground Masquerade," the purple mask worn by Manic and Max bares slight resemblance to NiGHTS into Dreams..., another game by Sonic Team.
- The episode "Sonic Tonic" recycles the general plot of the Adventures of Sonic the Hedgehog episode "Full-Tilt Tails."
- The siblings' father was never mentioned. However, the French theme song of the show implied that there was no king at the time that Sonic and his siblings were born.
- In episode 7, Sonic is holding a yellow controller similar to a Sega Genesis controller.
- In the episode "The Big Melt," Sonic and Manic are shown joining their force to do a huge version of the "Sonia Spin," a move that Sonic and Sonia were shown doing together before. However, as Manic doesn't have any physical superpower, he normally can not perform this move.
- The series was streamed on Netflix, but the episodes are completely out of order on Netflix and from episodes 27-40 the title screen for the episodes and the end credits are in French. As of 31 October 2013, Sonic Underground is no longer on Netflix. In 2014, Sonic Underground returned to Netflix.
- This is the first animated adaption to have animation done digitally.
The fourth wall is broken in this series.
- Throughout the show, Sonic and his siblings summon their band instruments and perform an episodic music video for the viewer. These segments often take place during an action sequence, prompting any onlookers to either stare in confusion or join the number.
all information on Sonic Underground came from http://sonic.wikia.com/wiki/Sonic_Underground