When Shredder teams with the Kraang to receive a batch of mutagen, the Turtles' problems only get worse--even putting their friendship with April at stake.

While tracking mutagen, the Turtles bring a mutated squirrel back to the lair. When it starts multiplying, the Turtles must hunt the squirrel mutants and prevent them escaping into the city.

When the Turtles face Shredder's newest threat, an army of Robot Foot Soldiers, Karai captures Leonardo and forces him to do battle with these new and improved ninjas that can adapt to his every move.

Donatello obsessively experiments with mutagen in a bid to find a cure for April's mutated father. Unfortunately this puts April in danger from a new threat--Mutagen Man!

Michelangelo is tired of being the "runt of the litter." But when he fools around with some mutagen, he gets a dangerous infection that is mutating out of control.

April tries to resume a normal teenage life, but when Karai starts hunting her again, April realizes she may have put her new friend Casey Jones in danger as well.

The Turtles find themselves being hunted by a new threat--another mutant turtle called Slash, who has a personal vendetta against the brothers.

After encountering mutants and ninjas, Casey Jones decides to take on "pure evil" alone. When he meets Raphael, the two hot-headed fighters must put their dislike for each other aside and work together against a common foe.

Feeling left out of the action, April insists on joining the Turtles on a mission to TCRI, where they learn a shocking secret about April's past.

A fungus mutant spreading through the sewers causes the Turtles and April to experience their worst fears as Leonardo must face his greatest fear--losing his team.

When Donnie upgrades Metalhead's artificial intelligence, he must decide if he can trust a sentient Metalhead... or if he has created a new foe.

The Rat King returns and tries to take control of Splinter again. Meanwhile, April leaves a cat with Mikey and he turns it into a mutant.

Leonardo considers asking Splinter for help while facing Shredder's new assassin; Donnie must put aside his rivalry with Casey in order to uncover a Kraang plot.

Facing Shredder's deadly new assassin, Leo must decide whether to risk his brother's lives or ask for Splinter's help. Donnie must put aside his rivalry with Casey in order to uncover a new Kraang plot.

When the Turtles play a fantasy LARP game to get their mind off of things, they must separate the difference between fantasy and reality.

The Turtles create a retro-mutagen to help April's father;Stockman-Fly kidnaps April.

Raphael is worried about Casey's ability to handle big mutant threats; Raphael and Casey cross the paths of Slash and his new partner.

Michaelangelo must battle Pizza Face, a mutant pizza that eats humans, in order to save his family.

When Tiger Claw returns, he vows to hunt and destroy the turtles. Meanwhile, Karai finds out the truth about her mother and real father, forcing her to make a life changing decision.

As Leo tries to convince the team to attempt to rescue Karai, Anton Zeck steals Shredder's helmet.

Leo stages a rescue for Karai against Splinter's wishes.

Donnie becomes increasingly jealous of April and Casey's relationship; April is kidnapped by an evil spirit who intends to drain her powers.

After Mikey gets trapped in Dimension X to try and save Leatherhead, his brothers come to his rescue, but they discover that their little brother has changed.

Leo and Donnie disagree about their plan to stop the Kraang invasion. When Leo makes a critical mistake, he is separated from the team and Donnie must step up as a leader.