Each student in Bart's class is given a video camera for a school project to create a movie. Otto drives the school bus to a drive-through restaurant where his girlfriend, Becky, works. Otto proposes to her, and she accepts. On Bart’s suggestion, they decide to have the wedding at the Simpson house and sends out flyers. Marge reluctantly agrees, since she still has everything from Apu's wedding. The wedding is a success up until the point when Otto gets a Poison tribute band (called Cyanide) to play "Nothin' but a Good Time". Becky admits to Marge that she hates heavy metal music. At the wedding, Marge suggests to Becky that she gives Otto an ultimatum: either her or heavy metal. Otto chooses heavy metal and breaks up with Becky.
Becky stays with the Simpson family on Bart's suggestion. Marge consoles her, but when Becky begins to help out around the house and is praised for her contribution, Marge begins to worry that her family likes Becky more than her. Eventually, she becomes paranoid and suspects that Becky is trying to kill her after her car won't stop on a steep road. Later, Becky and the rest of the family meet without Marge at an ice cream parlour, and Becky tells everyone that she has found a new apartment and is moving out. After she buys an unusually large bowl of ice cream, Homer freaks out upon the presence of so much ice cream and suddenly collapses. Becky gives him mouth-to-mouth but Marge comes in thinking she's kissing him. She grabs a cone and smashes it like a glass bottle to attack Becky. She is arrested and declared insane.
When Marge escapes from the courtroom and goes to the library to see who Becky really is, she realizes that she is insane and being unfair to Becky, after finding nothing bad about her and a newspaper tablet of how Marge ruined Becky's marriage. Finally, she returns home, only to find Homer tied up, Lisa's arms and legs stuck to the wall, Maggie stuck in a cage, and Becky holding a knife in her hands. When Becky is about to 'kill' Homer, Marge snatches the knife from her and attempts to strangle her. It turns out that Bart is filming a scene in his movie for the school (now a music video). Homer also reveals that while he was fixing Marge's car, he accidentally drained the brake fluid. Marge apologizes to Becky, but Becky admits she was going to kill Marge and steal her family, but decided not to after she couldn't get a shovel. Suddenly the mental hospital doctors show up and shoot three tranquilizer darts into Marge's neck, which don't take effect. Marge tells them that she has too much to do to take a nap. She instructs Lisa to take Maggie out of the cage and tells Homer that she has some 'S&M for him' - scrubbing and mopping, since he is dressed for the job anyway. As Marge giggles, Homer fires a dart borrowed from the doctors into her, which sends her straight to sleep.