This is a story about a young woman searching for the right man for herself. Anna Faris plays the character of Ally Darling. She reads a magazine article that says the average number of sexual partners that a woman has in her life is 10.5 and then when she adds up all the guys she has already slept with, her total is 19. The article also says that any woman that has slept with more than 20 men, they will probably never get married.
Ally goes out with her sister and their girlfriends to celebrate Ally's sister getting married. By playing a game with them, Ally finds out all the other women have slept with much fewer men than she has. Ally makes a vow to not sleep with more than 20 men, but gets drunk and has a fling with her ex-boss that just fired her. Now that she has reached the limit of 20 without getting married, she decides to go back to see if any of the men she knew from before might work out since they were all part of the first 20.
Ally finds out about them with the help of her over-sexed male neighbor Colin Shea (played by Chris Evans) that she helps get away from women he dates as a one-night stand in exchange for his help tracking down her old boyfriends. They find out most of her old boyfriends are either married, still not marrying-quality material (for example one turns out to be gay), or otherwise unavailable, except for one final one named Jake Adams.
Colin and Ally fall for each other. Ally gets Colin to take his relationships with women more seriously. Colin shows appreciation for Ally's artwork. They go out for a romantic evening, sneak into a sports arena, play strip "horse" basketball, almost get caught by security, then run off and go skinny-dipping in the harbor. Even though they had a really great time together, Ally's friends convince her that Colin is a loser. Colin and Ally have a fight. They stop seeing each other.
Ally goes out on a date with Jake Adams. He is rich, handsome, and she thinks he is Mr. Right. Ally takes Jake as her date for her sister's wedding. Her sister is freaking out because their parents are divorced and their father is bringing his new girlfriend to the wedding which makes their mother furious. Ally's mother wants her to marry Jake but when Ally goes with Jake to the wedding she realizes that it is Colin that she really loves. She leaves the wedding reception to reunite with Colin. Later she finds out from one of her ex-boyfriends that they actually never had sex only passed out in bed together so in Colin she actually does find the right guy for her, as her number 20.
Best part of story, including ending:
It is fun to see the relationships between Ally and all the men she met in her life.
Best scene in story:
The panic between all the girls helping prepare Ally's sister for her wedding when the wedding planners saws he will play the horn because the harpist is not coming is hilarious.
Opinion about the main character:
Ally is too easily influenced by the opinion of others but in the end she finds a way to have what she really wants.