After receiving a considerable donation of money, the people of Springfield decide what to spend it on. Enter Lyle Langley, a jocular salesman who gets Springfield hooked on a monorail system. After the monorail is up and running, and with Homer as the conductor, it's time for the maiden voyage. Little do the people know they have just boarded a one-way train to Hell.
- Mr Burns has been hiding toxic waste throughout Springfield, but is finally caught in the act, and forced to pay a $3million fine.
Springfield then holds a town meeting to decide what to do with the funds. Marge's idea to use the money to fix Main Street is quickly trumpeted by the townspeople, when a loud whistle is heard, and the townspeople's attention is drawn to a man named Lyle Lanley. Lanley's proposal is to use the money to fund a monorail system that he is proposing to have built. Lanley then breaks into a song that quickly turns the townspeople's votes in his favor.
As the town works on the monorail project, Marge is the only one who is still skeptical about the whole thing. Homer meanwhile, decides to apply for the job of a monorail operator, and is chosen by Lyle. When Homer gives his family an early glimpse of the monorail, Marge is shocked to find a family of possums in the place where a fire extinguisher should be. She goes to tell this to Lyle, only to find in his trailer a notebook featuring rough drawings of Lyle making off with the town's money while the monorail goes out of control.
The next day, Marge drives to the town of North Haverbrook, where Lanley had informed the town that his monorails had put them on the map. Instead, Marge finds the town desolate, and the monorail broken. She also encounters a man named Sebastian Cobb, who Lyle hired to build the monorail for the town. Cobb explains that corners were cut everywhere by using bad wiring and faulty brakes.
Marge and Cobb quickly head back to Springfield, but are too late to stop the inaugural launch of the monorail. Shortly after taking off, the monorail's equipment malfunctions, and it begins careening around the track at close to 200mph. Unfortunately, due to the train being solar-powered, shutting off the power won't help. Marge manages to radio Homer, and Cobb explains that an anchor is needed to stop the train. using the 'M' on the side of the monorail and a lasso, Homer manages to stop the out-of-control train.
Meanwhile, Lyle Lanley's flight takes a detour through North Haverbrook, where the townspeople quickly board the plane and beat up Lanley.
The show concludes with Marge explaining that the monorail was the last 'folly' Springfield ever did...except for the popsicle-stick skyscraper...and the giant magnifying glass...and the escalator to nowhere.