When Lisa becomes the target of a female bully, Lisa discovers a scientific reason as to why bullies pick on nerds. Meanwhile, Homer goes into the baby-proofing business, which is threatening the businesses of pediatricians and greeting card salesmen.

Homer develops a snoring problem, so Marge decides to spend a night with her sisters Patty and Selma. After a night of drinking, Marge sees a news report about her ex-prom date Artie Ziff, who is now very wealthy, and decides to send him an e-mail. Artie is still obsessed with Marge, so he offers the Simpsons $1 million in exchange for Marge spending a weekend with him.

Lisa tries to fit in with two college students by lying about her age. She finds that the college atmosphere is perfect for her, but her lie is soon discovered and she is shunned by her fellow elementary school students. Meanwhile, Bart is diagnosed with a weakened immune system after getting bitten by a Chinese mosquito and must live in a plastic, germ-free bubble.

Moe Szyslak becomes lonely and decides to commit suicide by jumping off a bridge. However, he inadvertently saves Maggie Simpson when she is sent flying from the Simpsons' car window and into his arms. He starts playing with Maggie and becomes attached to her, telling her the story of The Godfather. Homer and Marge become upset about how much time Moe is spending with Maggie and forbid him from seeing her again. One night, Maggie wakes up and hears Fat Tony and the Springfield Mafia outside the house and decides to follow them. When Homer and Marge find her missing, they automatically assume that Moe kidnapped her. When Moe is told that Maggie is missing, his request to help find her is granted by Homer. Maggie follows the mobsters to Luigi's restaurant, where Fat Tony's gang gets into a gun battle with a rival family. Homer, Marge and Moe track Maggie down and Moe decides to go inside and save Maggie. After Moe saves Maggie's life, Homer allows the two to spend more time together.

Homer builds a battle robot to win Bart's respect, and places it in a tournament against other robots. What Bart does not know, though, is that the robot is actually Homer in costume. Meanwhile, Snowball II dies and Lisa tries to find a replacement cat.

Bart gets sent to juvenile hall after fooling everyone that there was a marriage concerning him. He encounters a dangerous girl there... He flees with Gina, but then the two are caught by Wiggum.

Marge finds Nelson and acts as a mother figure towards him due to her children's loss of interest in her. Meanwhile, Lisa gets teased about her big butt, and becomes obsessed with her weight.