When Robotnik announces he plans to marry Queen Aleena, the shocked sonic siblings race to the rescue, unaware it's a trap..
Argus, the former captain of the Queen's guard, arranges a secret meeting with her. But he is soon intercepted by Robotnik's bounty hunters, Sleet and Dingo, who have every intention of feeding the soldier to the roboticizer, the Sonic trio race to the rescue..
Evading sleet and dingo, the hedgehogs fins themselves in a town completely untainted by robotnik's hand. Where the people are kind-hearted, and the food is free! Sounds like an ideal place, but there is more to Mobodon than meets the eye, after using the power stone, the siblings discover that this is where they were born! Before further questions can be answered,Sleet and dingo attack with an army of SWATBOTS, to preserve their past, the hedgehogs fight for Mobodon's future..
Sonia gets down 'n' dirty with Sleet and Dingo (literally), and to freshen up, she and her brothers journey to East Mobius. There, Sonia is reunited with some old friends, Mindy Latour and her father. Mindy gives Sonia a locket, but unbeknownst to Sonic's sister or Mindy, the locket is a tracking device designed by Robotnik. Sonia finds out too late, and soon Mindy's kingdom is swarming with similar wristwatch-style robots, all programmed to capture the hedgehogs! Can the siblings stomp out this bug problem?
Manic begins to backslide to his old ways when the SoUnd encounter, a group of teenage pickpockets, led by Max. Meanwhile, Sleet and Dingo are plotting to avoid giving the taxes they are collecting to Robotnik. They only need someone to frame for a theft, and Manic is the perfect victim.
Sonic meets up with his friend Cyrus. While Sonic believes that Cyrus has joined the Freedom Fighters, he is actually working for Dr. Robotnik. When he finds out that the Freedom Fighters' "sanctuary" is for children he destroys Robotnik's communicator. Sonic, Sonia and Manic try to get him away from Robotnik.
On a trip to Port Mobius Sonic, Manic and Sonia are told that ships are sinking due to the Mobian sea-beast Moby Deep. The hedgehogs also meet Captain Squeege who informs them of a scam in which the townspeople must rely on Robotnik for protection from the sea monster. In reality it is Robotnik who is sinking the ships. Sonic is urged to overcome his fear of water and the monster to find the truth and end the fears of the seaside town.
The Sonic Underground arrive in Mobia in search of the Queen's Journal. In the city Sonia loses her memory and meets Raphi, one of the town's resistance...
The hedgehogs come across Lake Valley, a beautiful resort. But how can it be beautiful when Robotnik's controlling the rest of Mobius? The band's about to find out...
Sonia has been invited to a debutante ball, arranged by none other than Robitnik's right hand man Bartleby! Sonic helps her in getting there without being caught, but even more problems arise when Cyrus chooses her to fly the captives of the ""Predator"" to safety...
Robotnik has created a super-fast robot, and he's organised a race for the top athletes of Mobius and the robot. But it's all a trap to drive Sonic out into the open... can he resist the competition?
Sonia inherits a spooky castle, but all is not as it seems when they stay there overnight...
In order to make a lot of money, Robotnik sells artifacts to the nobility of Mobius. But these artifacts are actually from more recent history; they are bombs, and they're about to go off.
The band finds a village that has been destroyed by its Freedom Fighter occupants. Why has this happened? It seems to be something to do with the Flybots all over the desert, as Manic finds out when he's injected with a mind-control toxin.
Using the essence of a rare plant, Robotnik manages to make Sleet and Dingo fast-- maybe even faster than Sonic. The Sonic Underground manage to get hold of it, but can Sonia and Manic resist the temptation of taking the drug themselves and being as fast as Sonic?
The band visit the Floating Island in search of their mother. There they meet Knuckles the Echidna, the island's guardian, who has been told by Sleet and Dingo that the Sonic Underground is after the Chaos Emerald.
Queen Aleena sends her children over to Speedster Island, a place Bartleby is trying to turn into a holiday resort. Sleet and Dingo are pretending to help, but they just want to robotocise the islanders, who worship three stone heads that resemble Sonic, Sonia and Manic.
The oracle, wishing to test the Hedgehog's practical capabilities, strips them of their powers and sends them into an alternate, roman-themed reality where a Sleet-look-alike presides as King. The former fast and furious hedgehogs must use their usual blend of cunning and strategy to save this reality, but without their super-speed, will they be stopped dead in their tracks?
Aleena's Royal Jewel can trace the Queen like a homing device.... now, I wonder who might be interested in that? As the siblings follow Mother and Robonik follows them, the elusive Queen seems almost within reach at last. Unless, of course, it's all an elaborate bluff.
While in Robotropolis, Sonic & co. find an abandoned baby hedgehog. While Sonic and Sonia search for its parents, Manic is left to mind it, and get in touch with his mothering instincts....or maybe just take it flying with him :) But the baby isn't as innocent as it looks...and Robotnik comes within an inch of uncovering Sanctuary.
The Sonic Underground come across a nomadic tribe whilst journeying through the desert. Manic claims to not trust the tribe's leader "Ifyoucan". He sets off to find out the truth and uncovers a shocking secret.
After stealing back a book engraved with the royal seal from Robotnik, the sonic trio discover that a prophet had foreseen Robotnik's takeover of Mobius, and has also foreseen his defeat! To even more surprise, it is revealed that the prophet also wrote down what he had foreseen on a scroll, which sonic and his sibling's quest for. The journey takes them through the treacherous Mobius desert [sandopolis or whatever you want to call it] and many perils and traps. But the underground prevail and locate the temple where the scroll was long hidden. In the meantime, sleet and Dingo, also find the temple, and pretty soon, the stage is set for a battle to decide the future of mobius..
In this episode the Sonic Underground rescue a prisoner from one of Robotnik's old prisoners - a hedgehog wearing a mask (so you can't see his face properly). The prisoner claims he is the hedgehogs uncle, the twin brother of Queen Aleena! But it is all just a sinister plan by Robotnik to split the Sonic Underground up.
In this episode, the Sonic Underground go and see the Oracle (again), who sends them to an alternate dimension, where they are the bad guys, and Robotnik is the good guy! But once Robotnik is on their side, they may be able to overthrow the bad hedgehogs and restore Mobotropolis to its former glory!
Robotnik creates a new flying fortress to lay waste to the planet. When the Sonic Underground suspects that the Chaos Emerald is keeping the fortress aloft, they head to the Floating Island to enlist the help of Knuckles. But we all know how hard-headed Knuckles is...
Queen Aleena tells the story of how Dr. Robotnik took over, Mobius, and how she had to give up her children [Manix, Sonia, and Sonic] to help fulfil a prophecy exclaiming that one day, the siblings will be able to rise against Robotnik's rule and restore peace to the planet..
After taking off with the Chaos Emerald, Sleet and Dingo have a quibble over it, ending with Dingo accidentally breaking it and unleashing all the chaos energy. The Sonic Underground and Knuckles are off to talk to Knuckles' grandfather Athair for advice on handling the situation. But with all the chaos energy tearing apart the planet, can they survive long enough to make it? And even if they do, will they listen to the wise echidna's words?
The newly reunited Sonic Siblings must learn to work as a team if they are to do battle with Robotnik. But do they really know each other well enough?
Knuckles bargains with Robotnik in order to recover and repair the Chaos Emerald before Mobius is destroyed, but to acquire the villain's help, the echidna must betray the Hedgehogs..
Sonic, Sonia and Manic put their training to the test as they embark on a mission to rescue the Oracle from Robotnik! Can they work together in harmony, or will their lacking seal their own doom?
The Sonic Underground are on a mission to stop a group of hillbillies from feuding so they can stop Robotnik's dam-building project from flooding the countryside.
A salon robot is hypnotising people that go to her salon into revealing deep secrets. When Sonia goes, she spills information about the resistance! Can she and her brothers keep Robotnik from getting the information?
A Mobian named Titus creates a machine that can de-robotisize robots, called, appropriately enough, the De-Robotisizer. But when Robotnik and his crew go after it, it may do more harm than good.
Sonic and Manic have been captured and are about to be robotisized! It's up to Sonia to save her brothers. But the thing is, they're in seperate robotisizers at opposite ends of the city!
Robotnik comes up with his most original plan yet: melt the polar ice caps. Things will be hard for the Sonic Underground when Sonia gets split up from the group and mistaken for Queen Sauna.
Robotnik invents a new dart that induces sleep and sends them after the resistance. It's up to Sonic and sibs to stop them from the source.
Bartleby is placed under arrest for helping out the resistance. As you might expect, it's the Sonic Underground to the rescue!
The Sonic Underground come across ART, Robotnik's most powerful robot ever, that can think and learn for itself. But ART soon learns the power of friendship...
Sonia finds a magic pendant that threatens to turn her invisible.
Sonic and Manic are introduced playing "Castle Conquest", a game of virtual reality, in order to enhance their pilot skills. After the two brothers are trapped in the game by Robotnik, it's up to Sonia to play the game to save her brothers.
