The Simpsons have dinner with Aunt Selma and her new boyfriend, Sideshow Bob, Bart's arch-enemy. According to Bob, while he was in prison he spent every moment planning his revenge on Bart for exposing his plan to frame Krusty the Clown; after receiving Selma's response to his "Prison Pen Pal" ad, he was inspired to become a model prisoner and earned an early release.
Bob proposes to Selma and she accepts. He makes an appearance at a Krusty the Clown telethon and makes amends; Lisa encourages Bart to forgive Bob, but he refuses to believe he has changed. When Selma discovers that Sideshow Bob detests her beloved MacGyver, the marriage is nearly called off until he agrees to Homer's suggestion to take a walk when Selma watches it.
Selma reveals that she is unconcerned about money, as she made a good profit in the stock market; Bob tells her he hopes he is not marrying her for her money. She also reveals that she has no sense of smell or taste and has cut back on cigarettes, now smoking only after meals and after episodes of MacGyver. Selma sends the Simpsons a tape of their honeymoon, including Bob's tirade over the absence of a gas fireplace in their hotel room. Bart realizes that Selma has one hour to live and the Simpsons rush to the hotel room.
In the hotel, when Selma retires to watch MacGyver, Bob enjoys a drink downstairs. The hotel room explodes behind him. He goes back to the wrecked room, assuming Selma is dead, but she is unscathed and the Simpsons are waiting for him along with the police. Bart explains how he deduced Bob's plot: Bob opened the gas valve in the room knowing Selma would not smell the leak, then left while she watched MacGyver, knowing that Selma would light a cigarette afterwards and cause an explosion. Bob asks why the room still exploded if Bart foiled his plot. Chief Wiggum explains that he absent-mindedly threw a match into the room after smoking a celebratory cigar. Bob is taken away by the police, vowing revenge.