After being caught by the Environmental Protection Agency dumping nuclear waste in the city park, Mr. Burns is fined $3 million. A town meeting is held so that the citizens can decide how to spend the money. Marge suggests that the city use the money to fix Main Street, which is in poor condition. The town is about to vote in favor when someone new to town, a fast-talking salesman named Lyle Lanley, suggests that Springfield construct a city monorail. After being swayed by a song, the now enthused townspeople decide to build the monorail.
Even though Lanley succeeds in winning over almost the entire town, his salesmanship fails to convince Marge, who is frustrated with the town's purchase because she correctly believes the monorail is unsafe and that Lanley is a conman. While watching TV, Homer sees an advertisement that suggests he attend Lanley's institute of monorail conducting, which is a transparent effort to make even more money from the con. Homer immediately decides to enroll. After a three-week course, Lanley selects Homer at random from among his classmates to be the monorail conductor.
Still annoyed about the town's lack of understanding of the monorail, Marge visits Lanley to question his motives, and discovers a notebook containing drawings which reveal Lanley’s intention to run off with bags of money skimmed from the monorail project while everyone else falls victim to his faulty train. Marge immediately drives to North Haverbrook, which Lanley mentioned was a previous purchaser of one of his monorails. She discovers that the town is in ruins. While exploring, Marge meets Sebastian Cobb, the engineer who designed Lanley's North Haverbrook monorail. Cobb explains that Lanley embezzled construction funds through shoddy workmanship and materials, and that the entire project was a scam. Realizing Marge believes him, he offers his assistance in helping to prevent the same fate from happening to Springfield.
At the maiden voyage of the Springfield monorail, Lanley arranges for a well-attended opening ceremony, which will divert the town's attention while he escapes on a plane to Tahiti. The whole town turns out, and Leonard Nimoy is the guest of honor. The monorail departs just before Marge and Cobb arrive. Although it runs normally at the start, the controls soon malfunction and cause it to speed wildly around the track. Homer, Bart, and the passengers are in danger, but the monorail's electricity cannot be shut off because of its solar power.
Meanwhile, Lanley's flight to Tahiti is interrupted by a brief and unexpected stopover in North Haverbrook. The townsfolk are alerted to his presence and they storm the plane to attack Lanley as revenge for ruining their town. Back in Springfield, Marge and Cobb contact Homer by radio and Cobb tells Homer that he will need to find an anchor in order to stop the train. Improvising quickly, Homer pries loose the giant metal "M" from the logo on the side of the monorail's engine, ties a rope to it, and throws it from the train. Eventually the "M" catches on the sign of a doughnut shop and the rope holds, stopping the monorail and saving its passengers.