- Directors: Adam McKay
- Producers: Jimmy Miller, Judd Apatow, Adam McKay, Will Ferrell
- Writers: Screenplay, Adam McKay, Will Ferrell, Story, Adam McKay, Will Ferrell, John C Reilly
- Genres: Comedy
- Actors: Will Ferrell, John C Reilly, Richard Jenkins, Mary Steenburgen, Adam Scott, Kathryn Hahn, Rob Riggle
Brennan Huff (Ferrell) is an unemployed 39-year-old who lives with his divorced mother, Nancy (Mary Steenburgen). Dale Doback (Reilly) is an unemployed 40-year-old who lives with his widower father, Robert (Richard Jenkins). Robert and Nancy get married and move in together, forcing Brennan and Dale to live with each other as step brothers; their childish dislike for each other is immediate.
Dale warns Brennan not to touch his drum set, but Brennan does so anyway. As tensions escalate, Brennan rubs his testicles (described by Ferrell in the DVD audio commentary as “$25,000 of prosthetic balls”) over Dale’s drums. This ignites a huge fight, in which Dale and Brennan use weapons such as headphones, bicycles, and bare hands to attack each other. In response, their parents declare that their sons must find jobs and see therapists or else be forced out of the house. Brennan’s successful, conceited biological younger brother Derek (Adam Scott) comes to visit with his oddly perfect family (who nearly cause a car accident as they sing an a cappella rendition of “Sweet Child o’ Mine” on the drive over). Dale retreats to his tree house where he and Brennan read pornography. Derek drops by to mock them, and incites Dale to punch Derek in the face. Brennan is awed by the fact that Dale was able to stand up to Derek. Brennan’s sister-in-law, Alice (Kathryn Hahn), meanwhile, finds Dale’s courage arousing, and becomes infatuated with him. Brennan and Dale become best friends.
Dale and Brennan go on to run a successful company that sells karaoke machines, and Brennan enters into a relationship with his therapist Denise (Andrea Savage). Robert and Nancy reunite and move back into their old home, with a new tree house made from the destroyed boat just for Dale and Brennan.