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Conclusion. The Kraang invasion is in full effect, and The Turtles fear there is no way to stop it.
Part 1 of 2. The Shredder forms a deadly alliance with The Kraang to assist in the destruction of Splinter, The Turtles, and New York City.
When The Turtles discover Leatherhead is trapped in Dimension X, Mikey makes a daring attempt to rescue him solo.
Leonardo conducts one final attempt to rescue Karai, against Splinter's wishes.
A Kraang mind-swapping machine becomes damaged during a Turtles rescue attempt, causing one of the Turtle's minds to become switched with a Kraang.
While attempting to rescue Karai, the Turtles are caught up in the middle of a master thief's plan to steal Shredder's helmet.
Tiger Claw returns to New York, and is out for revenge against the Turtles.
Mikey's brothers don't believe him when he says that citizens are turning into zombies because of living pizzas.
Raphael begins to question Casey's ability to help the Turtles in battle when they learn that Slash has joined forces with The Newtralizer.
When Donnie is finally able to create a Retro-Mutagen to cure April's father, the Turtles must risk losing it when a now mutated Baxter Stockman kidnaps April and holds her for ransom.
The Turtles take a break from their mutagen hunt to play a board game called Mazes and Mutants. Until a sparrow wizard named Sir Malachi brings their game to life.
When Donatello's brothers question Metalhead's newly advanced A.I., Donnie must decide if he can truly trust his mechanical friend in battle.
Raphael's pet turtle, Spike, is exposed to mutagen and changes into a hulking brute. He then sets out on a vendetta against Raph's brothers.
April still refuses to have anything to do with the turtles. However, an attack by the Foot clan may force her to change her mind.
Mutagen Man (the mutant version of the Pulverizer) escapes Donatello's lab and has his eyes set on April.
Chaos ensues in the lair when a mutant squirrel transforms and multiplies into dangerous Squirrelanoids.