"Black Widower" is the twenty-first episode of Season 3.
Sideshow Bob gets paroled and marries Selma. However, Bart suspects that Sideshow Bob has something else up his sleeve.
The Simpsons are having dinner with Selma and her new boyfriend, but it turns out it is Bart's adversary, Sideshow Bob. During dinner, he tells the story of his terrible time at the prison. He wins an Emmy that gets taken away and tells that he wanted to kill Bart, but also that he changed. Then he tells the story of how he and Selma met, which was during a prison pen-pal program. He supposedly turns a new leaf after he met Selma. He is then released from the prison, and the story ends. Bart refuses to believe that Bob has gone good. Then Bob asks Selma to marry him, and she eagerly accepts his proposal.
Sideshow Bob makes an appearance in the Krusty telethon, and he tells Krusty that there are no hard feelings. Bart is terrified of his soon-to-be new uncle, since Bob swore he would get revenge on Bart for having him sent to prison for framing Krusty for armed robbery, and he would never trust him. The wedding is almost canceled when Bob admits that he hates Selma's favorite show, MacGyver, but is saved by Homer when he mentions that when Marge watches her shows, he goes for a walk or drinks some beers and comes back still in love.
During the wedding, everyone is happy except Bart. During the honeymoon, while MacGyver is on, Sideshow Bob tries to kill Selma, but his plan is ruined by Bart, who makes a "stinkin'" discovery, as Bob referred to it. Bob is taken away by the police, vowing to return the next time the Democrats are in power. After that, everyone thanks Bart for not losing his mistrust of Sideshow Bob.
Behind the Laughter
How Bart uncovered the trap
In Selma's video, Bob was eager to have a fireplace in their room. Bart noticed the gas tap. Selma didn't smell the gas as she had lost her senses of smell and taste due to a bottle rocket accident as a child.
Selma mentions to Sideshow Bob that she made a good fortune on the stock market, which gave Sideshow Bob the idea to kill her in hopes of inheriting her money.
Selma promised at the wedding to give up smoking except for after meals and after MacGyver. The spark from a match was enough to make the room explode.
Bart was able to save Selma before she could light up her match. The explosion that Bob heard was Chief Wiggum absentmindedly tossing a match he lit up for celebratory cigars into the hotel room.