Harry Sanborn (Jack Nicholson) is a perennial playboy with a libido much younger than his years. During what was to have been a romantic weekend with his latest infatuation, Marin (Amanda Peet), at her mother's Hampton's beach house, Harry develops chest pains. He winds up being reluctantly nursed by Marin's reluctant mother Erica Barry (Diane Keaton), a successful, divorced New York playwright. In the process, Harry develops more heart pangs - but this time the romantic kind - for Erica! However, some habits die hard. When Harry hesitates, his charming thirty-something doctor (Keanu Reeves) starts pursuing Erica and it is Harry who must undergo a true change of heart if he is to win her back.