500 Keys
Season 22, Episode 21 • Original airdate: 2011-05-15
Milestone anniversary: 15 years
When the Simpsons discover a collection of keys to every door in Springfield, Lisa stumbles upon an eerie hidden classroom beneath Springfield Elementary School. When she shares her discovery with Principal Skinner, the secret room mysteriously disappears and he takes the only key away. A determined Lisa uses her detective skills to lead her back to the room to solve an old school mystery.