500 Keys

"The children are on Bus 23"

- The words a "mysterious figure" writes on the chalkboard in the hidden classroom.

"500 Keys" is the twenty-first episode of season 22.

Plot
Returning from a vendor that sells returned wedding cakes, Maggie gets locked in the car. The Simpsons (while searching for a spare key) discover a collection of keys that'll open every single door in Springfield. Lisa uses one of the keys to open the door of a hidden classroom, which is full of props underneath the school. Intrigued about the discovery, she informs Principal Skinner of the hidden room. It becomes apparent that Skinner and Superintendent Chalmers are hiding something and they take the key from Lisa. However, Lisa has an identical key made and returns to the door. She sees a mysterious figure writing "The children are on Bus 23" on the chalkboard. Lisa then becomes extremely determined to solve the mystery of the hidden classroom.

Meanwhile, Bart tries to be bad by causing mayhem with the keys, but he accidentally does good deeds with them, and ends up with the key to the city. Marge and Maggie find a key for a wind-up toy train called "The Pooter Toot Express", which makes farting noises when it moves. The toy gets away from them and they chase it throughout the city. Elsewhere, Homer uses a key to get into the Duff Beer brewery and goes joyriding in a blimp he finds. Lisa asks Nelson Muntz about the mysterious Bus 23. Nelson tells her that Bus 23 was a bus full of school kids that was supposed to drive over a dangerous ice bridge but was never heard from again, as Skinner and Superintendent Chalmers covered up the loss of the children. Later on, Lisa tells Bart, and the two ask Homer to take them (in the blimp) to where the ice bridge would be. On route, Lisa falls out of the blimp into the water of the lake, where she stumbles across Bus 23. She discovers that all the "children" inside the bus are actually mannequins. "The Pooter Toot Express" knocks over a nearby tree which Lisa uses to grab onto. She learns that the mysterious figure (who was seen writing on the chalkboard) is Otto Mann. Otto is relieved to discover that he didn't kill any of the children when he was driving Bus 23.

Chalmers and Skinner are about to flee to Bolivia, but are stopped by Bart. The pair finally explain about everything about Bus 23 and the hidden classroom: Apparently the school received a lump sum of grant money to improve its classrooms, but when Skinner cashed the check, the money was accidentally destroyed in his laundry by his mother. Chalmers and Skinner used Willie to help build a fake classroom, rented mannequins and posed them like students, and then photographed everything to fool the government into thinking they were spending the grant money on the school. Immediately afterwards, they put the mannequins on Bus 23 to return them (to avoid being charged an extra day's rental fee), but the ice bridge gave out and the bus fell into the lake. Back in the present, after telling the story, Skinner and Chalmers apologize to Otto for letting him believe he killed a busload of students, and all is forgiven.

Epilogue
The episode ends with Otto driving his bus across an icy bridge, believing that the vehicle is full of mannequins, when he is actually driving real children home. His bus is close to tipping off the bridge and he comments that perhaps they will fall off slowly like in the movie . Otto is wrong, however, and the bus falls quickly into the river.

Production
It was written by John Frink and directed by Bob Anderson and contains a small cameo in the opening sequence by Albert Brooks as Hank Scorpio, who last appeared in the eighth season episode, "You Only Move Twice". Also, the chalkboard gag was written to correct the error from the last episode "Homer Scissorhands": guest star Kristen Schaal's surname was spelled incorrectly as "Schall" in the credits.

Reception
"500 Keys" was viewed by an estimated 6 million households and received a 2.5 rating/7% share, remaining even with the previous episode. The episode received positive reviews from critics, with Hook Hougan (The A.V Club) commenting that the episode was "amusing" but "never comes near great stuff". He concluded with giving the episode a B grade. It was also positively received by fans of the show who complimented the fun mystery element of the storyline with many calling it one of the best of the season.

When this episode premiered on Sky1 on October 30, 2011, it received 1,014,000 viewers and was ranked 3rd that week.