“The Why of Fry” (Season 4, Episode 10)
Unlike The Simpsons or, really, any other animated sitcom, Futurama had an ongoing story and lore that the writers built up over the years through tiny clues and foreshadowing. And in this season 4 episode, many of these various plot points and hidden clues get tied up and resolved. For example, remember when Fry had sex with his grandma and became his own grandad? Funny, but also, it apparently changed his brain and made him immune to the attacks of a race of brain-destroying aliens.
