As Principal Skinner plays with a platoon of plastic army men, threatening them with a stapler, Chalmers enters the room and announces that it’s time to assign the school’s vending machine contract. After meeting with various snack representatives, Skinner and Chalmers settle on the “Scammer and Z-Dog” machines, which have a hip-hop theme. The machines are a big hit with students, and Bart becomes addicted to the “scarf-able” meals. Even at the dinner table, Bart eats “scarf-ables” instead of Marge’s home cooking. Before long Bart puts on weight. When Homer introduces Bart to the brass slug, which takes the place of pocket change, it unlocks an endless supply of junk food, causing Bart’s weight problem to get worse. In a spoof of the show’s opening credits, an obese Bart rides his skateboard home from school. Only this time, at the end of the sequence, Bart staggers to the couch, clutching his chest.

Bart is rushed to a hospital, where Dr. Hibbert connects Bart to a heart monitor sensor and urges him to relax. When Bart returns home he exploits his medical condition, getting others to perform the simplest of tasks for him. He also ignores his diet and gobbles down junk food. The family decides there’s only one thing left to do: stage an intervention. When Bart finds out what’s going on, he makes a break for the front door. He’s apprehended by two orderlies, thrown into a van, and driven away.

Bart is taken to a health spa called Serenity Ranch. Faculty director Tab Spangler sets to work, escorting Bart to a group session attended by other people with a weight problem; including Apu, Rainier Wolfcastle, and Kent Brockman. Marge thinks up a way to pay for the expensive health spa bills: she turns the house into a youth hostel. Before long it’s overrun with German backpackers. Tab Spangler realizes Bart’s treatment isn’t going well. He decides to take the boy for a drive. They eventually stop in front of the Simpson house. When Bart realizes his home has been invaded by demanding Germans, he vows to swear off junk food for good. He returns to school, and fire axe in hand, breaks into the “Scammer and Z-Dog” machines. When Bart shows up at his house, he carries with him bulging sacks of change. He tells his family he went to the bank and withdrew all his savings so the family wouldn’t have to struggle with health-spa bills. With Bart back home, Homer drives the Germans out of the house. Tab Spangler shows up and reminds the Simpsons that they still have three weeks left on a non-refundable weight-loss treatment. Homer wonders if anyone close to the family can benefit from the program. Tab realizes Homer doesn’t know he’s overweight. He drives Homer to the health spa.