After collecting a $3 million fine from Mr. Burns (for illegal disposal of nuclear waste), Springfield holds a town meeting to decide what to do with the money. A smooth-talking stranger named Lyle Lanley talks the townspeople into spending the money on a needless monorail...built from shoddy materials.
At the Nuclear Power Plant, Homer lays asleep in his office. When it's now the end of work, Homer dances his way to his car and drives home. On the way home, Homer starts singing a Flintstones song saying, "Simpson. Homer Simpson. He's the greatest guy in history. From the town of Springfield, he's about to hit a chestnut tree." At that moment Homer then crashes into a chestnut tree.
After being caught dumping nuclear waste in the city park by Agent Malone, Mr. Burns is fined three million dollars. A town meeting is immediately held so that the citizens can decide how to spend the money, and Marge suggests using it to fix up Main Street, which is in extremely poor condition. Abe Simpson tries to protest fixing Main Street; however, the townspeople interrupt him and they mistakenly think that Abe is favor of the idea. The town shows enthusiasm for this idea and is about to vote for it when suddenly a smooth-talking stranger named Lyle Lanley suggests that the town buy a new monorail, making his pitch in a song which convinces the town to buy the monorail.
Marge is unhappy with the town's purchase, believing that they should have fixed Main Street and that the monorail is unsafe. While watching TV, Homer sees an advertisement that suggests he become a monorail conductor. Claiming it to be a lifelong dream, Homer signs up for the monorail conductor training course. After a decidedly unintensive three weeks of training, Homer is named the monorail conductor.
Still feeling uneasy about the monorail, Marge decides to visit Lyle Lanley and discovers a notebook that reveals Lanley’s true intentions: to have the monorail break down while he gets away with the town's money. Lanley catches her, and Marge quickly says that she was just browsing around. Believing her story, Lanley allows her to leave, not knowing that she has read the notebook and is aware of his plans.
Marge immediately drives to North Haverbrook, which Lanley mentioned was a previous purchaser of one of his monorails. She finds the town to be run-down and nearly deserted. Marge asks around about the monorail, but no one is willing to talk about it: a woman working at a drive-in diner says there's no monorail in North Haverbrook and never was, but then closes the doors, revealing that the diner is called "MONORAIL CAFE". Finally, Marge meets Sebastian Cobb, who explains that Lanley cut costs everywhere while building North Haverbrook's monorail, and that the entire thing is a scam. Cobb then shows her the remnants of the town's monorail, which is now broken and in disrepair.
At the maiden voyage of the monorail, the entire town turns out, including celebrities such as Lurleen Lumpkin and Leonard Nimoy. Lanley grabs his money and jumps in a taxi, which takes him to the airport. The monorail takes off just before Marge and Cobb arrive in Springfield. At first the monorail runs smoothly, but the controls malfunction, causing the monorail to speed up and travel at a dangerous speed. Nobody can figure out how to stop the monorail (since it's solar-powered) and although a solar eclipse briefly halts it, the eclipse ends and the monorail takes off again. Meanwhile, Lanley’s flight makes a brief unscheduled stop in North Haverbrook, where Lanley is immediately attacked by a group of locals who have been tipped off about his arrival, perhaps by Marge. Back in Springfield, Cobb tells Homer that in order to stop the train, he needs to find an anchor. Homer grabs the giant “M” from the side of the monorail, ties it to a rope, and throws it out the window (just barely remembering at the last second that he needs to also tie the rope to something inside the monorail). Eventually, the M latches onto a giant doughnut, stopping the monorail and saving its passengers. As the passengers get off of the monorail, Leonard Nimoy says that his work is done. Barney tells Nimoy that he didn't do anything. Nimoy asks, "Didn't I?" and vanishes in a transporter effect very similar to Star Trek.
The episode ends with Marge explaining that the monorail was the last folly Springfield ever had...except for the popsicle-stick skyscraper...and the giant magnifying glass...and the escalator to nowhere.