In his lab, Baxter Stockman is attacked by the Purple Dragon street gang and a large amount of his mutagen is spilled down a drain. This leads to the creation of four mutants in the New York sewer, who are then raised and trained as ninjas by their rat sensei Splinter. Some time later, Stockman reappears using an exosuit to take control of the Purple Dragons and terrorize the city, so the Turtles set out to battle the Dragons and Stockman, while also saving kidnapped news reporter April O'Neil.