Teenage Mutant Ninja Turtles (1987): Season 9, Episode 8

9 - 8: Doomquest

An interdimensional being called Doomquest comes to Earth to try to get a powerful crystal from Lord Dregg. The turtles find that their unstable mutations are now starting to cause them to lose their intelligence whenever they mutate into their new forms. In the end, April is able to successfully expose Lord Dregg's true intentions to the world.