Ally Darling, a young woman struggling with her romantic life, reads a magazine article suggesting that women who have had 20 or more lovers are unlikely to ever get married. Counting her relationships, she realizes she's already reached 20. In a panic, Ally revisits her past relationships to see if any of her ex-boyfriends have significantly improved, hoping she can reignite a romance rather than surpass her critical number. Her neighbor, Colin Shea, a carefree, womanizing musician, offers to help her track down her ex-lovers in exchange for letting him hide out in her apartment to avoid his own romantic entanglements.

Using Colin's skills and resources, Ally begins reconnecting with her ex-boyfriends, each encounter highlighting the humorous and disastrous reasons why the relationships failed. As she learns more about herself and what she's truly looking for in a partner, Ally's perspective on love and relationships evolves. Meanwhile, Colin's unexpected support and charming but sincere demeanor lead to a growing bond between them. Through a series of comedic and heartfelt events, Ally discovers truths about her past relationships and begins to see her current life and potential future in a new light.