Melvin Udall (Jack Nicholson), a successful, but misogynistic writer with obsessive-compulsive disorder, likes to control every last detail of his life. He has stringent rules about what small things, such as what sort of pavement he can or can't walk upon, but it is his insistence on receiving his breakfast only from waitress Carol Connelly (Helen Hunt) that changes his life. He finds himself adrift one morning when she is absent from work, forcing him into the sort of close contact with the people around him that he's made a science of avoiding. Yet another strange series of events leaves a dog owned by his neighbor, Simon Bishop (Greg Kinnear) in Melvin's care. Soon, the three find themselves in an unlikely friendship, beginning an adventure that forces Melvin's highly structured life to change entirely.