The entire Simpson family stays at home during a blizzard. The weather is too wild to play outside and the power goes out, so Bart participates in a pretend fashion show put on by his sisters, Lisa and Maggie. When he notices the bond shared by the two girls, Lisa suggests that he's jealous because he does not have a brother. That night, Bart dreams about being in a park (called Bro-Town USA) with the Smothers Brothers, the Marx Brothers, the Blues Brothers, the Smith Brothers, the Wright Brothers, the Mario Brothers, the Mannings (Cooper, Eli and Peyton), and even the Terwiliger brothers, (Sideshow Bob and Cecil). Bart awakens with the realization that he wants a brother.
After getting turned down by Homer (who gives a lengthy monologue detailing why daughters are better to have than sons), Bart consults with his friends at the bus stop. Nelson advises Bart to trick Marge and Homer into having a baby the same way Nelson's mom tried to trick former basketball player Charles Barkley. Bart's first attempt is to make them a romantic dinner, but Marge and Homer are too stuffed to feel amorous. In his second attempt, Bart leaves a kama sutra DVD in his parents' bedroom, but the two end up injuring themselves after emulating the sex positions. Frustrated, he seeks the advice of Dolph, Jimbo, and Kearney, who tell him to hide Marge's birth control pills. Bart replaces Marge's birth control pills with Tic Tacs (just as he did with Mrs. Krabappel's in "Itchy & Scratchy: The Movie") and discards the Tic Tacs container that now has the birth control pills inside, which Nelson finds and eats (giving the bully mood swings due to the estrogen ingestion). Marge catches Bart tampering with her pills and he confesses that he wants a brother. Despite feeling bad for Bart, Marge explains that she and Homer are fine with just three kids and also points out that even if the family did end up having another baby, the baby may end up being another girl instead of a boy, resulting in Bart having three sisters. Bart decides to go to the local orphanage, but he's turned away due for being too young to adopt a child, but a young boy named Charlie follows him home.
Bart and Charlie do brotherly things together, including playing pranks on Principal Skinner and hanging out at the Kwik-E-Mart. When Lisa insists that Charlie be returned to the orphanage, Bart disregards her and takes Charlie to see a horror movie titled Sever V. The movie terrifies Charlie, which makes Bart realize that being an older brother requires responsibility. On their way back from the movie, Chief Wiggum tries to apprehend Charlie, but the two boys escape and hide in a snowbank. Back in the snowbank, Lisa pleads with Bart to do the right thing and let Charlie be legally adopted by another family. When the children are sealed in by a snowplow (by Barney riding on his Plow King truck), Bart and Charlie urinate on the snow to melt it and open an escape tunnel, much to Lisa's disgust. Charlie is later adopted by a family with six girls, who all treat him like their personal fashion doll much to Charlie's dismay. To cheer Bart up, Homer takes him to see Sever V.