Damon makes a stupid bet that he can get a girl to say yes to a proposal within a month. He meets Melanie and decides that it is her he must convince. What he doesn't know is that Melanie left her three previous boyfriends because she didn't want to commit to them and she has now decided to resist new boyfriends.
I'll admit that I didn't have high expectations for this movie. The story sounded like it could derail into boyishness etc. quite thoroughly. But judge by my surprise that they actually managed to make a pretty good romantic movie out of it. Even if the end is a bit implausible.