Molly (Kate Hudson) and Carl (Matt Dillon) are preparing for their wedding day in Hawaii, until Carl's friend Neil (Seth Rogen) interrupts to say that Dupree (Owen Wilson) got lost. They drive off together to pick up Dupree, who appeared to have hitched a ride with a light plane after landing on the wrong island. A day before the wedding, Molly's father Thompson (Michael Douglas), who is also CEO of the company that Carl works for, makes a toast with humorous remarks at Carl's expense, foreshadowing a conflict between the two. Later at a Pre-celebration at a bar, Carl neglects Dupree to be with Molly while he is about to perform a drinking tradition. Carl and Dupree later make up on the beach, as Dupree apologizes for laughing at Molly's father's jokes, and tells Carl that he has "Carlness." Carl and Molly get married.When Carl returns to work, at Molly's father's Thompson Land Development, he is surprised to find that Mr. Thompson has promoted him to be in charge of a design he proposed, though it had been altered somewhat. Mr. Thompson makes absurd requests which proceed to get worse, starting with Thompson's drastic re-imagining of Carl's new architecture project, requesting that Carl take the Thompson name instead and that Carl get a vasectomy to prevent childbearing later on.Before returning home to celebrate his promotion with Molly, Carl stops by the bar, where he finds Neil and Dupree. After Neil leaves due to his wife's curfew, Dupree reveals that he was fired, as he apparently did not have clearance to attend the wedding, subsequently falling behind in his rent and evicted. Losing his car also, as it was the company's. Carl takes Dupree home, asking Molly if he can stay for a little while until he gets back on his feet. Molly is polite, though clearly frustrated as he is disruptive and messy.Molly sets up Dupree with a woman at her work, a primary school, who is a Mormon librarian. Dupree agrees, though Molly is shocked to find them together when she opens her front door coming home from dinner. Romantic candles burn down the front of the living room and Dupree is evicted. Meanwhile Carl is being continually stressed out from work, though he and Molly find time to go out for dinner. On the way back they find Dupree sitting on a bench in heavy rain with his belongings. Molly insists they take him back in. Carl makes it known that Dupree must behave this time. The next day, Dupree makes amends, refurbishing the living room, and doing Carl's thank-you letters, as well as making friends with kids from the block. Carl asks Dupree to go to career day at Molly's work as he got tied up at his work and that night Dupree cooks a large dinner for Molly, though Carl is late again, so Molly and Dupree start without him. When Carl finally shows up, he is a little jealous that they were having dinner together, and have a fight.Carl kicks Dupree out, suspecting an affair, which shocks Dupree. The following night, Mr. Thompson is over for dinner. Dupree climbs the drain pipe and sneaks inside to get his bags back, and during dinner the guests inside hear Dupree fall off the roof. Dupree is found outside and is invited in for dinner. After Mr. Thompson takes a liking to Dupree and asks him to go fishing with him, it enrages Carl, who jumps across the table and attempts to strangle Dupree; Thompson hits Carl over the head with a candlestick shortly after. After returning from the hospital with a neck brace, Dupree and Molly confront Mr. Thompson about what he really thinks of his new son-in-law, while Carl had left. The next morning Dupree gets all the local kids to search for Carl. Dupree eventually finds Carl in the bar, and convinces him to chase after Molly. Dupree helps Carl break in to Mr. Thompson's office, as Dupree distracts Paco, the security guard, whilst Carl marches into his father-in-law's office and confronts him. The two finally reach an understanding and Thompson admits to his agenda of insulting Carl. Dupree and Carl return to the house, where Carl and Molly reunite, Carl apologizing, and agree to work it all out.Dupree becomes an author and a motivational speaker, with Paco now at his side as his number two.