The Simpson family goes to Rainier Wolfcastle's bankruptcy garage sale. Homer asks Rainier if he has anything that will increase in value when he dies, and is shown an old weight-lifting set, complete with dumbbells and bench press. Homer takes it and packs everything and everybody in the car in the style of a Tetris game. The car ends up being so overpacked that Homer has no room in it for himself and ends up being carried home by Rainier Wolfcastle in a baby harness built for an adult.
On the way home, Marge and the kids discover that Maggie has soiled her diaper. Marge pulls into the Kwik-E-Mart, and changes Maggie in the restroom. As she is leaving the store, a shady character in a Goofy hat accosts her and threatens her with a gun. Finding only diapers in Marge's purse, he grabs Marge's pearl necklace and takes off. Marge, stunned, walks to her car in a daze and breaks down sobbing at the wheel in front of her kids.
The next day, they inform the police and Chief Wiggum declares he will investigate immediately. Later, as Marge is about to leave the house, Homer gives her some pepper spray and some tips on how to handle an attacker. She pulls up to the Kwik-E-Mart, but she snaps when Ralph greets her. She pepper-sprays him on impulse and, feeling guilty, drives back home where she feels safe. When she reaches home, Bart tells her she is parked over the mailman. Marge is a nervous wreck and cannot bring herself to cross the threshold of her house to help the mailman. Dr. Hibbert diagnoses Marge with agoraphobia. Homer and the kids try their best over the next few days to encourage her to go outside, but to no avail. Eventually, she moves into the basement. There, feeling a bit safer, she prepares breakfast for the family and sets up a bed for herself. One day, when she is alone at home, she eyes Rainier Wolfcastle's weight-lifting set and decides to use it to pass the time.
In two weeks, she builds herself up and even gets a well-defined washboard stomach (much to Homer's delight). She dashes out to the garden to get some lemons and, realizing that she is not afraid anymore, starts running around town. She runs into her mugger; even though the mugger does not move a finger against Marge, she beats him to a bloody pulp out of revenge in a scene that references the 1972 film The Godfather, when Sonny Corleone beats his brother-in-law Carlo Rizzi. The cops arrive and arrest the crook, and Marge starts exercising even more. One day, as she jogs by an open-air gym at the beach, she runs into Ruth Powers, her old neighbor (seen in episodes "New Kid on the Block" and "Marge on the Lam"). Ruth is also very muscular, and tells Marge that she owes it all to steroids. She talks Marge into using them, and also advises Marge to enter a women's bodybuilding contest. Using the steroids, Marge exercises harder than ever, becoming very muscular and estrogen-depleted, which results in a short temper and a violent streak. Homer, seeing these changes, finds himself less attracted to her. When Homer refuses Marge's sexual advances the night before the contest, she callously ignores his worries and then proceeds to pin him down and rapes him, before leaving him completely sore and exhausted to take care of the kids the next morning.
That night, the family attends the Iron Maiden Fitness Pageant to see the women's bodybuilding final. Marge wins second place, which irritates her when her muscular ears overhear her family's unhappy conversation in the audience. Later that night, at Moe's, Marge boasts to the all male attendees about her performance at the competition. Homer then tells her he is proud of her ability to bulk up but not lose her femininity. Marge is angered by that, saying that was the reason she came in second place. She then tells everyone at the bar that she intends to up her glycol-load, use a denser ripping gel, and that she didn't sacrifice her period to come in second place. Moe says to Marge, "I don't got enough booze to make you look good". She flies into an uncontrollable rage and ultimately trashes the bar. Homer confronts his wife as she prepares to hurl Lenny. Terrified, Homer tells Marge that he misses the sweet, feminine woman he married. Marge, horrified with what she has done, apologizes, drops Lenny, and leaves with Homer. In order to cover the costs of repairing his bar, Moe sets fire to it; however, Carl points out that he has not insured the place yet.
Later, at the Simpson house, Marge burns the weight set in the furnace as the show ends. Homer asks if Marge is ready for a "real workout" which turns out to be a request to wax the car, after which he gets beaten by Marge into meekly saying that he was just kidding.