Episode 36 of "Futurama"
s03e07 The Day The Earth Stood Stupid

The Professor informs the crew that their next delivery is no longer necessary because that planet has been destroyed. Meanwhile, Leela enters Nibbler in a pet show and wins dumbest pet. The evil brains responsible for destroying planets have arrived on earth and start making everybody stupid. Nibbler sees this and jumps on his spaceship and takes Leela to his home planet. Nibbler then informs Leela about his race and the evil brains. He tells her that Fry is the only hope because he is immune to the brains effects. Leela then tells Fry he must defeat the main brain. He finds the brain in the library and tricks the brain into thinking he must leave Earth. Nobody but Fry ends up remembering any of what occurred - except for Nibbler.