For some reason, we find Homer and the kids looking at returned wedding cakes (like from the interracial wedding reception where the bride's father made a racist joke). The end result is the Simpsons discovering that they have the keys for every lock in Springfield. Giving them that much access to anything is never a good idea, and some long-sunken secrets are about to come to light thanks to the "500 Keys".
While Bart's mischief spree yields unexpected results and Homer commandeers the Duff Blimp, Lisa is tempted to use the Springfield Elementary master key to poke around. Deep in the bowels of the school, she finds a multi-disciplinary classroom, preserved in pristine condition, but with blank maps and unmarked rulers. Principle Skinner denies the room's existence, which mysteriously vanishes when Lisa attempts to unveil it to her fellow students. The only way to crack the secret of the classroom, and the missing Bus #23, might be Homer's anniversary present to Marge.
It actually makes for an interesting change to have a Simpsons episode title that isn't a riff on a popular culture trope (unless I'm missing something here). Surprisingly, despite all the main characters having their own story, every angle works: Homer's shenanigans with the Duff Blimp are priceless and Lisa's genuinely interesting quest are the clear winners. The incredible journey of the Pooter Toot Express is less of a success (although maybe that's because toilet humor has never really worked for me - no, really), and Bart's failure at causing anarchy thankfully didn't last long - although it was worth it for the absolutely horrifying umbilical cord gag. I watched that scene twice and cringed both times.
That every one of those stories has its own defining moment, and that they're all naturally resolved by the end of the episode, solidifies "500 Keys" as a great story - which is no small feat after last week's "Homer Scissorhands". None of the characters go through any major life changes that we know will be reset by the end credits, so we can enjoy everything that happens without cheap tricks or being led by the nose.
Only Marge and Bart's stories lag, but even they have their good moments. I won't go over the umbilical cord thing again (you can thank me later), but the Pooter Toot Express shaming Ned Flanders and causing an irritable bowel syndrome support group to go into fits of irritability is always good.
In the end, it's Homer, as usual, who walks away with the best moments of the episode. Whether it's cleaning Maggie by sticking her head in his mouth, leading the Springfield Police Department on a wild blimp chase, telling Bart to "Fly the blimp, you spoiled kid!" and calling him "a real jerrrrrrrk!" as he plummets into a lake, Homer carries the episode. I should complain, since the main story is about Lisa and the secret classroom, but everything Homer does is so damnably funny that "500 Keys" wins overall.
4.5/5.0: Homer alternately laughing and crying at the self-destruction of the Pooter Toot Express. That's why I watch this show.