While in Shelbyville, the Simpsons watch a musical about the town, which includes a character from Springfield who isn't particularly bright. Marge suggests to the Springfield Cultural Activities Board to have a concert hall, built by Frank Gehry (voicing himself); however, when it becomes clear that nobody in Springfield likes classical music, the building is sold to Mr. Burns, who turns it into a prison. One of its first inmates is Homer, who is arrested for "abusing litter" when he kicks a can five times, in violation of a decades-old law. When Snake tries to escape, Homer thinks he "accidentally" fell into a garbage truck and tells the guards; Mr. Burns, the warden, thinks Homer has a future as a snitch - and so does Homer, when he gets a large-screen plasma TV for his help. Once word gets out, however, a phony jailbreak is leaked to Homer so the guards won't be around when the prisoners go after him...
Meanwhile, Bart and Lisa's attempts to discover why Snowball II is so fat lead them to a shocking discovery; their cat is leading two lives by living with another family - and is able to do tricks for them as well.
While the family visits Shelbyville, the citizens of Shelbyville call Springfield's residents "hicks." This angers Marge, who suggests to the Springfield Cultural Advisory Board that architect Frank Gehry design and build a new cultural center for Springfield. Impressed with Marge's suggestion (which she printed on stationery with Snoopy), he submits his own design which the town approves. The $30 million project is eventually finished and it opens and closes swiftly because no one in Springfield likes classical music (past the first few bars of Beethoven's 5th).
This allows Mr. Burns to swoop in with an idea of his own: take over the space and turn it into a state prison. Although Homer applies for a guard job, he fails the guard test because Otto switches his urine sample (loaded with so many drugs there are only trace amounts of human urine) and Homer's sample. Burns wants Chief Wiggum to bring back some old and forgotten laws in order to fill his prison with convicts. After Homer kicks a can 5 times in a row, he is thrown into jail for illegal transport of litter.
Homer goes to jail, and when he is there, he (unknowingly) snitches on Snake's escape attempt. Burns makes Homer a prison snitch. Without a doubt, Homer snitches up and gets rewarded by the guards, even getting a new plasma TV. During a visit from Marge she loudly exclaims that he was the snitch while other prisoners are getting visits as well. Fat Tony and his goons want to learn about the snitch and tell Homer about a breakout. The guards are waiting outside for the breakout, but a riot from the prisoners who want to get Homer breaks out. It is stopped, with Homer snitching about the prison's conditions. Eventually, he gets released and the governor releases all of the prisoners onto a barge ("where you will all bare-knuckle box until one of you emerges as king of your floating hell").
Meanwhile, Bart and Lisa find out that Snowball V has been gaining weight due largely in part to eating food from another family's house. They also reveal that she has another family. The new family adopts her and names her "Smokey". Smokey likes her new family, and Bart attempts to get her out, but the family fills up Bart with good food. At the end when Homer sneaks out to Moe's he sees Snowball V headed to the bar as well. And all he said was "I won't tell if you don't tell."
During the credits, we see Homer running into the concert, claiming the building is a death trap. But Bart says that Homer had fallen asleep in front of the TV watching The Towering Inferno and that his claim is only a dream. At that rate, Homer gets angry at Bart for knowing the name of his "prophetic vision".