Mitch Planko Jr. (Ryan Reynolds) is a drifter in life. A would be actor who swears, womanizes, smokes pot and doesn't finish things. He's best friends with Dave Lockwood (Jason Bateman). A successful lawyer, father of 3 and loving husband to Jamie (Leslie Mann).
They have a night out after Dave blows off his date night with his wife, again. After a few drinks they pee in a fountain in a park. The next morning they find they have switched bodies.
They go back to the fountain to find it moved. The city can't tell them where for 3 days to 3 weeks.
They try and live each other's lives but add their own flare. Mitch has to learn to be a father, husband and lawyer about to make partner by landing a big merger. Dave has to deal with booty calls, a lite porno shoot and no responsibility. After a few tutorials they get use to their new roles.
Eventually they realize their life wasn't so bad and want to switch back. They find the fountain in a crowded mall. They pee in the fountain and hope for the best. It worked and they are back in each other's bodies. Dave realizes that Mitch manscaped. Mitch realizes that Dave gave him a large back tattoo stating 'I heart Dave' with a picture on the date with Sabrina McArdle (Olivia Wilde), Dave's assistant.
The happy ending is one month later at Dave's anniversary after they both attend Mitch Planko Sr.'s (Alan Arkin) wedding.
Thanks Ann-Marie 'crappy' F!