Andie Anderson, a talented writer at Composure magazine, feels trapped by the monotony of penning formulaic "how-to" articles about inconsequential topics. Following her friend Michelle's latest breakup, Andie is inspired to confront more meaningful themes in her work. Determined to validate her theory that needy behavior leads to relationship failures, she devises an article titled "How to Lose a Guy in 10 Days." This leads her to embark on a daring experiment: she aims to woo a man with the express purpose of driving him away within ten days, mirroring the emotional challenges faced by her friend Michelle.
Meanwhile, Benjamin Barry, an ambitious advertising executive, is on a mission to expand his portfolio by leading a high-stakes campaign for an esteemed diamond company. While at a local bar, his boss, Phillip, doubts the team's ability to embody the romantic allure of diamonds in their advertising. In a moment of boldness, Ben wagers that he can make any woman fall head over heels for him within ten days, and if successful, he will land the prestigious diamond account.
As luck would have it, Ben's adversaries, Judy Spears and Judy Green, discover Andie at Composure magazine and realize she would be the perfect candidate for Ben's experiment. They reveal to Ben that Andie is unknowingly part of his wager. When their paths cross, both Andie and Ben harbor secrets they refuse to expose. Andie is determined to make Ben fall in love with her, only to break his heart later, while Ben remains surprisingly resistant to her strategies.
As the game heats up, Andie employs a series of outrageous tactics to push Ben away. This includes convincing him to skip an essential Knicks game, orchestrating a mishap in a movie theater, moving her things into his apartment, and overwhelming him with an array of pets—such as a fern and a Chinese Crested Dog. She even tricks him into attending a Celine Dion concert while claiming it's a Knicks game. Yet, despite her best efforts to undermine their relationship, Andie occasionally unveils her genuine self, unintentionally drawing Ben closer.
In a desperate attempt to sever ties with Ben, Andie creates a scene at his poker night, hoping to provoke a breakup. However, Ben's friends, Tony and Thayer, cleverly suggest couple's therapy as a solution. Determined to uncover Ben's true feelings, Andie recruits Michelle to pose as a therapist, leading to accusations that Ben is embarrassed to be with her. Undaunted, Ben proposes introducing Andie to his family on Staten Island over the weekend, and as they navigate this new dynamic, their authentic connection starts to blossom.
The company ball becomes the stage for their next encounter, and Andie and Ben make a striking entrance as a couple. As the night unfolds, Phillip reveals that Andie has genuinely won Ben's heart, while Judy and Judy share the grave news with Tony and Thayer that Andie was aware of the bet all along. Unbeknownst to them, this discovery triggers Andie's plan to sabotage the relationship.
As the evening progresses, Lana confides to Ben about Andie's article, inadvertently affirming Andie's suspicions. With a newfound sense of betrayal, Andie seizes the chance to confront Ben publicly, leading to a dramatic breakup that reverberates through their circle of friends.
The aftermath of their confrontation finds Tony presenting Andie's revealing article to Ben, capturing her profound loss over the only man she has ever truly loved. As news circulates that Andie is leaving Composure for a new opportunity in Washington D.C., Ben races to intercept her taxi, intent on discussing their feelings. In a heartfelt exchange, they reveal their true emotions, and Ben instructs the driver to return Andie's belongings to her home, declaring he will be joining her. As the taxi disappears into the night, they share a tender kiss, leaving behind a glimmer of hope and the promise of newfound love.
