When the turtles witness the kidnapping of April O'Neil and her father Kirby by the Kraang, Donnie urges them to rescue the duo.
The turtles locate the Kraang hideout where April and Kirby are being held prisoner, but freeing them means fighting Snakeweed, a giant mutant plant.
The turtles try to rescue an obnoxious New Yorker who has videotaped and threatened to expose them, but he is mutated into a spider villain.
To prove the boys can be friends with humans, Mikey goes online to befriend a martial artist — who turns out to be a pupil of Shredder's.
Grounded by Splinter, the turtles sneak out anyway and lose their new T-Pod, which a bitter former TCRI employee uses for his own vengeful ambitions.
Donnie creates a remote-controlled robotic turtle fighter named Metalhead to take his place, but the Kraang are able to seize control of it.
The case of a missing neurochemist leads Donnie and April to a psychic monkey, the bizarre result of experiments using mutagen and monkey DNA.
After the turtles battle the Purple Dragons at a noodle shop, Shredder sends the Foot Clan after them led by Xever Montes, a brutal street fighter.
The turtles and April set out to rescue her father and stop the Kraang from detonating a mutagen weapon in the city, but Shredder surprises them.
Terrified of Shredder defeating the turtles, Splinter puts the team into round-the-clock training, which pays off when Shredder attacks the sewers.
The turtles are forced to split up into two teams when both the Purple Dragons and Stockman's MOUSERS launch separate attacks.
After the turtles rescue the surly mutant alligator Leatherhead and Mikey nurses him back to health, they discover he has the Kraang's power cell.
While letting an increasingly critical Raphael try leading the group, Leonardo meets a Foot named Karai, who tries to get him to switch sides.
The team must save New York and Splinter from the evil Dr. Victor Falco, who has been transformed into the powerful Rat King.
Leo and Raph fight over whether they can trust Karai, while the Kraang are drawn to April's high school science project, leading to a violent fight.
As the Kraang search for their power cell, the turtles meet a fan: a teenager with no combat skills who dresses up as "The Pulverizer" to fight crime.
The turtles discover the location of TCRI headquarters and break in to discover the Kraang's plan: to open a doorway to Dimension X and invade.
Donnie creates a roach that's really a spy camera to keep tabs on the Kraang lab, but the "spyroach" becomes mutated and goes after Raph.
Stockman and Shredder lure the turtles, along with Dogpound and Fishface, into a labyrinth of lethal traps and snares called the Maze of Doom.
With a Kraang invasion imminent, Karai offers a truce and access to new weapons from Shredder's arsenal, but can the turtles trust their new ally?
The turtles infiltrate a submerged Kraang base and capture a droid. When the Foot learns that April is key to the alien plot, Karai races to detain her.
Shredder learns about the mutagen weapon, while the turtles persuade the Pulverizer, who has been recruited by the Foot, to spy on the enemy for them.
Attacked by an enormous mutant Kraang wasp, the turtles slowly come under its toxic spell and will do anything to protect the wasp's surviving egg.
Trying to impress April, Donnie breaks into a secret Kraang prison by himself to rescue Kirby and faces a deadly detainee called the Newtralizer.
The turtles launch an assault on TCRI to shut down the portal and prevent the Kraang from bringing the Technodrome through the portal.
The turtles must enter the Technodrome and face Kraang Prime in order to save April. In battle against Shredder, Splinter recognizes a familiar face.
When the Turtles try to keep a Kraang shipment of mutagen from getting to to Shredder, an accident causes consequences for April's father.
Karai takes charge of the Foot when Shredder leaves to find reinforcements and uses new weapons to seize Leonardo and fight those who rush to his aid.
When a squirrel forces its way down a man's throat and multiplies inside him, the Turtles link the bizarre event to the missing mutagen.
When Donnie tells Timothy about his jealousy over a student whom April tutors, the Pulverizer mutates into a blob and goes after April.
When Michelangelo contracts a menacing case of mutant acne, the Turtles must find a molecular centrifuge so that they can create an antidote.
April tries to mend her relationship with the Turtles when the Kraang and Karai come after her with a powerful new Footbot and his regiment.
Raph is thrilled when his pet turtle turns into the hulking mutant, Slash, who wants them to team up as ninjas, but Slash is hiding dark intentions.
While April insists on joining the Turtles on a special mission, an enigmatic figure appears with a surprising revelation about her origins.
Casey Jones meets with Raph and determines that he's a threat to New York, but when Raph tries to set him straight, a new menace arises at the lair.
April, Casey and the Turtles fall victim to a wave of mutant mushroom spores that invades the sewers and activates their deepest fears.
Metalhead becomes the subject of suspicion after his artificial intelligence gets upgraded, and the Turtles uncover a Kraang plot to control mutants.
Splinter is forced to face his fears when the the Rat King returns. Michelangelo adopts a cat that unwittingly eats ice cream containing a mutagen.
Leo may need Splinter's help when Shredder returns to town with a fearsome new trainee. The Kraang use giant worms to set off earthquakes in New York.
With earthquakes rattling the city, Leo, Raph and Mikey try to help Splinter, while Donnie and April search for a way to stop the gigantic worms.
When a mutant bird sends the Turtles into the world of a game they play for fun, they are forced to enter a maze and fight a dragon to rescue April.
The Turtles must choose between April and a mutagen that can restore Kirby to human form. Shredder punishes Stockman by turning him into a housefly.
When tensions flare between Casey and Raph, Casey gets a daunting chance to prove himself in a confrontation with Slash and his ominous new partner.
Tiger Claw enlists Karai in a ruse to find the lair and exact revenge on the Turtles, but when she learns the truth about Splinter, Shredder steps in.
Mikey becomes the only hope for his crew when they are turned into zombies by living pizzas who answer to a pepperoni and mushroom mutant.
Leo tries to redeem himself for losing Karai to Shredder and persuades the Turtles to help rescue her, using Shredder's valuable helmet as bait.
When one of the Kraang's infiltration mechanisms gets damaged, Raphael finds that he and a Kraang have had their minds swapped.
After Leo stages a mission to rescue Karai, her fury at Shredder boils over, causing her to fall into a terrible trap Shredder set for the Turtles.
Donnie and Casey must save the day when an evil spirit tries to sap April's life force and programs Mikey, Leo and Raph to be his soldiers.
When the Kraang perfect their mutagen, the Turtles travel through their portal into another dimension and battle rock monsters to rescue Leatherhead.
After arguing with Donnie over plans to stop the Kraang invasion, Leo makes an error that separates him from the team, so Donnie has to take the lead.
With the city under siege and the entire planet at risk, Leo, Donnie and the others square off with the Kraang.