3ACV07: The Day the Earth Stood Stupid

Leela, discovers that Nibbler isn't just a cute dumb animal, but an intelligent being, whose mission is to defeat the Brains from enslaving free thought. She goes to Nibbler's home planet and is told that the Brains make people stupider, but Fry is immune to such a change. She is sent back to help Fry succeed in his destiny of saving Earth; although she ends up, like the others, stupid.