When Lisa tells Bart that he will never have the special bond that she has with Maggie because he doesn't have any brothers, he tries to get Homer and Marge to make him a baby brother, but when that doesn't work, he goes to the orphanage---where a young boy follows him home.
The 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 Lisa and Maggie. When Bart notices the bond shared by the two girls, Lisa suggests that Bart is jealous because he doesn't have a brother. That night, Bart dreams about being in a park (called Bro-Town USA) with many famous brothers: the Smothers Brothers arguing, the Marx Brothers playing their trademark instruments, the Blues Brothers dancing, the Smith Brothers coughing, the Wright Brothers with their plane and bike, the MarioBrothers running into a tube, quarterbacks Peyton and Eli Manning, along with their lesser known older brother, Cooper (who currently works for an energy company in New Orleans), playing football, and Sideshow Bob and his brother Cecil flying kites. He awakes with the realization that he wants a baby brother.
After asking Homer for a brother and being refused (Homer explains that while he loves his children, he only intends to go to the hospital again when he dies, and also goes into a lengthy explanation of why daughters are better than sons), Bart consults with his friends, who advise him to trick Marge and Homer into making love (with the scene being a parody of South Park, even ending with Otto hitting Ralph and saying he killed Kenny, a character from the show that almost always dies in the first five seasons).
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 karma sutra DVD in his parents' bedroom, but they hurt themselves when they try unusual positions. Frustrated, he seeks the advice of Nelson and the other bullies, who tell him to hide Marge's birth control pills. He switches them with Tac Tics, a parody of Tic Tacs (just as he did with Mrs. Krabappel's, as mentioned in "Itchy & Scratchy: The Movie") and throws the Tac Tic container (which now contains birth control pills) out the window, where it is found by Nelson. Marge catches Bart tampering with her pills and he confesses that he wants a brother. Marge, though she feels sorry for Bart, explains that she and Homer are happy with just three kids and that even if they do ever end up having another baby, it might be end up being a girl instead of boy (resulting in Bart having three sisters instead of the brother he desires). Hearing that, Bart goes to the local orphanage, but is turned down because he is too young to legally adopt a child. However, a young orphaned boy named Charlie follows him home.
Bart and Charlie do brotherly activities together, including playing pranks on Principal Skinner and hanging out at the Kwik-E-Mart. When Lisa insists that Charlie needs to return to the orphanage, Bart disregards her and takes Charlie to see a horror movie entitled "Sever V". The movie terrifies Charlie, which makes Bart realize that being an older brother requires a lot of responsibility (which he never realized until then). On their way back from the movie, Chief Wiggum tries to apprehend Charlie, but the two boys escape and hide in a snowbank.
Meanwhile, Nelson has become increasingly emotional and weepy from eating Marge's birth control pills. 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 the snowbank by a snow plow, Bart and Charlie urinate out of the tunnels, much to Lisa's disgust. Charlie is later adopted by a family with six girls. Homer sees that his son misses Charlie, so the two watch a scary movie together.