The Turtles discover that the Kraang survived their previous showdown and have dropped mutagen canisters all over New York City.

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 from escaping into the city.

The Turtles face an army of robot foot soldiers; Karai captures Leonardo and forces him to do battle with ninjas that can adapt to his every move.

Donatello experiments with mutagen in an attempt to find a cure for April's mutated father. But one of his expirements goes wrong and he is unleashed in the city.

Michelangelo plays with some mutagen, contracting a dangerous infection that mutates out of control.

April tries to resume her normal life; April realizes that she may have put her friend in jeopardy when Karai starts hunting her again.

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.

April feels left out and insists on joining the Turtles on a mission to TCRI; A secret about April's past is revealed...

A fungus mutant spreads through the sewers causing the Turtles to experience their worst fears. Leonardo must face his greatest fear.

After Donnie upgrades Metalhead's artificial intelligence, he must decide if he can trust a sentient Metalhead.

When the Rat King returns, Splinter must overcome his fears in order to defeat his enemy.

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.

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.

When the turtles play a Fantasy LARP game to get their minds off things, they separate the difference between fantasy and reality.

The Turtles create a retro-mutagen to help April's father; Baxter Stockman threatens to mutate April.

Raphael is worried about Casey's ability to handle big mutant threats especially when they meet Slash again and discover he has a partner of his own, the Newtralizer.

Mikey struggles to convince the others that a new mutant is trying to attack and capture people using living pizzas.

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

Leo tries to convince the team to rescue Karai; a master thief steals Shredder’s helmet.

Raphael accidentally damages a machine that forces him to swap minds with a Kraang. Now he must find a way to convince his brothers and Splinter that it's really him, before the Kraang controlling his body destroys his brothers or worse, reveals their location to the rest of the Kraang.

Leo stages a rescue for Karai against Splinter's wishes. While the mission is a success, Karai's knowledge of the truth makes her rush out to take her revenge on Shredder for killing her mother and lying to her for so long, leading to an unexpected turn of events.

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

When the Turtles go to the bizarre realm of Dimension X, Mikey is the only one who can figure the place out.

Leo and Donnie disagree about their plan to stop the Kraang invasion; Donnie must step up as the leader of the bunch when Leo makes a mistake and is separated from the team.