Nelson Moss (KEANU REEVES) is a busy and self-centered, but successful advertising executive who, like his business associate, Vince (GREG GERMANN), doesn't have time for anyone else, let alone his girlfriend beyond a few moments of sex. When he has to take time out of his hectic schedule to renew his drivers license at the DMV, he runs into Sara Deever (CHARLIZE THERON) and then ends up being responsible for her losing her permit to drive for a month.
Although he offers to pay her for her troubles, Sara decides to leech onto Nelson, much to his dismay. Threatening to make repeated public scenes if he doesn't humor her, Sara gets Nelson to drive her around town and then return to her apartment where she offers to help him improve his life. The only condition is that he must move in with her for the entire month of November and do everything she says.
He thinks she's crazy and leaves, but when he's suddenly fired from his job and dumped by his girlfriend, he decides to return and the two end up in bed together. When he then meets Chaz (JASON ISAACS), a close friend of hers, and Abner (LIAM AIKEN), a young, fatherless neighbor boy, and both wonder if he's this month's "flavor," Nelson realizes it's time for him to leave.
Sara eventually convinces him, however, that he has nothing to go home to and that she has her reasons for her apparent promiscuousness, and so Nelson stays with her. As they spend more time together and Nelson soon begins to loosen up and fall for her and her laidback and seemingly carefree ways, he must choose between his old and new lifestyle all while dealing with an unexpected revelation that challenges his convictions.