Advice columnist Dan Burns (Steve Carell) is a widower with three daughters, whom he brings to his parents' vacation house for a family get-together. When Dan goes to a local bookstore, he meets Marie (Juliette Binoche). To catch her attention, Dan poses as a shop clerk, and convinces her to buy a random selection of books. When Marie finds out the truth, Dan proposes to make it up to her. The two share an intimate conversation and Dan starts falling for her. Marie, who gets a phone call and starts to leave. Dan asks for her number, but Marie refuses, saying that she has just started a new relationship, but then relents and gives Dan her number.
On his way back to the house, Dan, an otherwise careful driver, is so engrossed in the memory of Marie that he runs a stop sign and gets a ticket. When he gets home, he finds out to his surprise that Marie is his brother Mitch's (Dane Cook) girlfriend, but not before he mentions that he met a girl. Dan tells Marie that they should not mention the bookstore incident to the family. The family starts liking Marie, while Dan gets envious and starts to behave strangely. Over the next few days, the two go back and forth in forbidden love and a few laughs.
Dan's mother Nana (Dianne Wiest) announces the she ran into Ruthie Draper (Emily Blunt), an old acquaintance of Dan's, and that she has set up a date between Dan and Ruthie. Dan recalls that when they were kids, Ruthie's nickname was "Pigface" and refuses to go. Mitch proposes that they double with him and Marie, and Dan agrees. While Ruthie turns out to be very hot, Dan isn't too interested. However, on Marie's indication that Ruthie is not right for him, Dan feigns interest and starts dancing with Ruthie. Marie gets jealous and joins the floor with Mitch, leading to an upstaging contest between Dan and Marie.
During a game of family football, Dan continues to flirt with Marie, and as a result, gets hit on the nose by the ball. His eldest daughter, Jane (Allison Pill) confronts him and tells that she has noticed his flirting and warns him to stop. Just then, Dan gets a phone call and finds out that some representatives from a syndicated publication will be coming to interview him for a position.
The night before the interview, the family has a talent show, where Dan accompanies Mitch singing a love song to Marie. Mitch forgets many of the words, and Dan ends up finishing the song for him. In the morning, Marie, torn by the pressure of falling for Dan, dumps Mitch and leaves. She calls Dan, and he leaves to meet her, running the same stop sign and getting a second ticket from the same cop in the process. They meet up and decide to go bowling. In the meantime, the representatives have arrived and the family comes to look for Dan, only to find the two sharing a kiss.
Dan explains, saying that Marie was the woman he met at the bookstore. Mitch hits Dan. Marie runs out and drives away. Dan tries to go chasing after her, but rams the same cop's car while backing out and loses his license. After this, Dan realizes he's been neglecting his family and kids. He admits to his family and potential employers that he's been violating his own advice and ignoring his family, and apologizes to his kids. He tries to apologize to Mitch, only to find out that Mitch is now going out with Ruthie. Dan's daughters realize that he is in love with Marie, and they encourage him to go find her. They then go to New York to find Marie, take the job, and live happily ever after.