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500 Keys is the twenty-first episode of Season 22. It first aired on May 15, 2011.
Synopsis
Lisa tries to return a key to Springfield Elementary, but she stumbles upon a abandoned classroom in the basement. She attempts to tell Skinner about the room, but when she returns, a bookcase blocks the dusty walls and the cobwebs leading to the room. Lisa becomes suspicious about a writing on the blackboard which involved Bus 23, a former bus used by the school on a field trip that never returned. Lisa and Bart, who has just won the key to the city, team up with Homer, who has stolen the Duff blimp, to try and find this mysterious Bus 23, which is submerged below the icy waters of the river. Meanwhile, Marge is on a continuous search for her noisy anniversary present.
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