The Turtles comes into conflict with T.C.R.I. scientist Dr. Baxter Stockman and the return of their enemy, The Shredder, who has hired Stockman to create mutants of his own in the form of Bebop and Rocksteady to even the score, during which the Turtles discover a Retro-Mutagen in hopes they can become humans. Following an extraterrestrial invasion above New York City led by the Dimension X inhabitant known as Krang. Upon Krang's Invasion, the Turtles are joined in the fight with their Master Splinter, their human friends April O'Neil, Vern Fenwick and new friend, vigilante Casey Jones.