When Marge forces Otto to choose between rock 'n' roll and his bride to be, Marge quickly finds herself playing hostess to a now-single Becky. But when Becky's attempt to earn her keep upstages Marge's own homemaking abilities, Marge quickly finds herself losing her mind.
For a school project, each kid in Bart's class is loaned a video camera to make a movie with. Otto drives the School Bus to the drive-through where his girlfriend Becky works. He proposes to her, and she accepts. Following Bart's suggestion, they decide to have the wedding at the Simpson's house. Marge agrees, as they still have all the stuff from Apu's wedding. Otto hires a Poison tribute band named Cyanide to play at the wedding, at which point Becky admits to Marge that she can't stand Heavy metal music. Taking Marge's advice, Becky gives Otto an ultimatum; either her or heavy metal. The wedding is called off, the guests return the wedding gifts, and Otto drives off with the band in the School Bus.
Due to Marge ruining another wedding, Becky is allowed to stay with them for a while. The family grow to like the new girl, seemingly more than Marge. After her 'brakes cut light' starts blinking and her car fails to stop on a steep hill, Marge becomes paranoid, believing that Becky is trying to kill her and seduce Homer. She visits the police, where Chief Wiggum says he can't help her. Meanwhile, Becky and the rest of the family meet at an ice cream parlor (without Marge, as they could not find her). Becky tells everyone that she's found an apartment to live in, and buys everyone huge ice creams to celebrate her moving out. Homer is shocked by the sight of so much ice cream, and collapses on the floor. Marge walks in just as Becky is giving him mouth-to-mouth; believing they were kissing, she attacks.
Marge is arrested and taken to a sanity hearing. The board declare her insane, taking all her actions (including praying and her faith in God) as signs of madness. Before she can be committed, she escapes the courtroom, becoming a fugitive. Heading to the library to check up on Becky's history, Marge finds nothing bad about the girl, but a newspaper clipping about herself ruining Becky's wedding. She decides she is being paranoid, and returns home.
Upon entering the house, she finds Homer tied up, Lisa stuck to the wall, Maggie in a cage and Becky holding a knife, heading for Homer. Marge snatches the knife away and strangles Becky, before Bart reveals that this is all a setup for his school project (a music video).
Homer reveals that he had cut the brake line of Marge's car, accidentally while working on it. When Marge apologizes for her behavior, Becky admits that she was intending to kill Marge and steal her family, but gave up when she couldn't find a good shovel. Doctors show up and fire three tranquilizer darts into Marge's neck, which don't do anything. She orders Homer to do some scrubbing and mopping, as he is "dressed for the job anyway". In reply, Homer shoots a fourth dart into her neck, which makes her fall into a deep sleep.
The episode's ending was used by Chris Suellentrop of Slate as an example in his editorial about the show's decline in quality, from a realistic show about family life into a typical cartoon.
"But under Scully's tenure, The Simpsons became, well, a cartoon. ... Episodes that once would have ended with Homer and Marge bicycling into the sunset (perhaps while Bart gagged in the background) now end with Homer blowing a tranquilizer dart into Marge's neck."