Plot
Andie Anderson is a writer for the women's magazine Composure as the subject-matter expert for a series of "How to" articles. She is bored and wishes to write about more serious topics.
Andie's friend Michelle experiences yet another breakup and becomes despondent, saying she is not attractive enough. Andie argues that the problem is instead her needy, excessively emotional behavior with men, and is inspired to write an article, "How to Lose a Guy in 10 Days". To prove her point, she will begin dating a man and, by imitating Michelle's behavior, provoke him to dump her within 10 days.
Advertising executive Benjamin Barry is looking to branch out from his usual remit of beer and sports campaigns by leading a prestigious ad campaign for a diamond company. At a bar, Ben's boss, Phillip, questions whether Ben has enough insight into the romance typically associated with diamonds.
In response, Ben wagers he can make any woman fall in love with him. Phillip says that if he can achieve this before the company ball in 10 days, he will allow him to lead the diamond campaign. Ben's rivals, Judy Spears and Judy Green, were at the Composure magazine offices earlier in the day and know about Andie's project. Seeing Andie at the bar, they suggest her as the woman he romance, and he agrees.
Ben goes and talks to Andie and they start their relationship, neither revealing their true intentions. She works hard to get him to first fall for and then break up with her, while he sticks to his task. She makes Ben miss the final shot at a Knicks game by sending him to get her a soda, gets him punched in a movie theater by talking loudly while watching a film, moves very girly things into his apartment, gets him a fern plant to represent their love and a Chinese Crested Dog, and takes him to a Celine Dion concert instead of another Knicks game like he expected. But she also occasionally lets her normal side show, which Ben begins falling for.
Moving on, Andie behaves outrageously at Ben's boys' poker night and drives him to break up with her. However, Ben's friends Tony and Thayer push him to stay the course by proposing couple's therapy with Andie, so he chases after her and she reluctantly agrees.
Andie has Michelle pose as a therapist and together they criticize Ben, who counters by offering to introduce Andie to his family in Staten Island that weekend. When they visit, she finds she likes his family, and is moved when his mother hugs her and asks her not to break Ben's heart. Ben and Andie begin to form a genuine bond.
Andie and Ben go to the company ball together. After Phillip talks with Andie, he tells Ben that he "met her, she loves you, you win". To sabotage Ben, Judy and Judy tell Tony and Thayer that Andie knew about the bet all along and was playing along to help him win.
Tony and Thayer ask Andie to keep quiet, unwittingly making her aware of the bet. Simultaneously, Andie's boss Lana, unaware of Ben's role, tells him about Andie's article. Having learned of Ben's bet, Andie attempts to humiliate Ben in front of everyone at the party, but he responds in kind and the pair argue on stage and break up.
Tony shows Andie's subsequent article to Ben, in which she writes how she "lost the only guy I've ever fallen for". When he goes to see her, he discovers she quit her job at Composure (because Lana again refused to let her write about serious topics) and is on her way to Washington, D.C., for a job interview.
Ben chases her taxi and gets Andie to stop. After he says he has read her article and accuses her of running away, they reveal their true feelings for each other and kiss.
