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What "The Simpsons" 21-th episode about
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.
Episode number: 21
Episode name: "500 Keys"
Episode release date: 16 may 2011 at 00:00
Status: aired
Rating: 7.3