Brennan Huff (Will Ferrell) and Dale Doback (John C. Reilly) are two grown men who still reside at home, stuck in a state of immaturity. Brennan lives with his divorced mother, Nancy (Mary Steenburgen), while Dale is under the roof of his widowed father, Robert (Richard Jenkins). When Robert and Nancy fall in love and decide to marry, both Brennan and Dale are less than thrilled, resulting in them being forced to coexist as stepbrothers.
Their initial interactions are marked by contempt, as both despise the other's intrusion into their lives. A critical moment occurs when Dale warns Brennan not to touch his beloved drum kit, but Brennan disregards the warning. This leads to a heavy confrontation, culminating in a chaotic brawl between the two. After this incident, they are grounded without access to television for a week, and their parents impose an ultimatum: they must find jobs within a month or face eviction.
Nancy recalls an embarrassing moment from Brennan's teenage years when he participated in a talent contest only to have his brother Derek (Adam Scott) sabotage him by bribing the choir. Meanwhile, the successful Derek’s visit further aggravates the situation, as he openly mocks their lack of ambition and dreams, winning their father Robert’s admiration in the process. This leads Dale to punch Derek, momentarily solidifying a bond between the two stepbrothers.
As the story progresses, Brennan and Dale begin to discover their common interests, from identical dinosaur favorites to the same magazine for personal gratification. In a twist of fate, Robert arranges job interviews for both, but they only fail to impress. With their living situation at risk due to plans to sell the house and retire on a boat, they become determined to sabotage Derek’s real estate efforts.
In their desperation, they decide to launch an entertainment enterprise named "Prestige Worldwide," harnessing Brennan’s singing talent and Dale’s marketing flair. Their first presentation at Derek's birthday party is disastrous, showcasing a video of Robert's boat crashing, which devastates their parents' sailing dreams. When financial losses and angry confrontations ensue, Robert expresses his desire to divorce Nancy, prompting a breakdown in their relationship dynamic.
In the weeks that follow, they manage to find independent jobs, trying to adjust to adult responsibilities. However, Brennan, wishing to mend family ties, takes on the task of organizing a significant event—the Catalina Wine Mixer—for Derek's company, inviting Robert and Nancy to rekindle old flames. Surprisingly, the evening goes awry when the band's lead singer is ejected, creating a chance for Brennan and Dale to reclaim the spotlight. They seize the moment and perform "Por Ti Volare," a powerful display that not only strengthens family bonds but also helps them restore their unique identities.
By the end of their journey, six months have passed, and Robert and Nancy have reconciled, returning to their former home. Brennan and Dale have transformed "Prestige Worldwide" into a thriving karaoke entertainment company, showcasing their resilience and creativity. In a whimsical turn of events, Robert has even turned his boat into a treehouse in the backyard, proving that their eccentricity can indeed lead to happiness.
