In Boston, the promiscuous Ally Darling is fired from her job in a marketing company. When her sister Daisy is arranging her wedding party, Ally reads an article in a women's magazine stating that women that have slept with more than twenty guys usually do not get married. Ally counts her affairs and realizes that she has slept with exactly twenty boyfriends. Ally decides that the next man that she sleeps with will be her husband. Ally decides to seek out her former boyfriends and asks her next door neighbor, the womanizer Colin Shea, to help her to find them to check whether any of them has improved his life. Ally and Colin become close friends. Meanwhile Ally meets Jake Adams, who was her sweetheart in high school and now is a successful businessman. She believes she has found her Prince Charming.