Writer Melvin (Jack Nicholson) is an old man with a weird temper and obsessive-compulsive disorder. He has no family or friends. He stays in his house all day to write his 62nd novel. His life is as precise as a clock and he can't stand any interruption. Melvin's neighbor Simon (Greg Kinnear) is a young gay painter who has a cute puppy. He is lively and mild-tempered and is often insulted by Melvin. Melvin only goes out once a day to eat at the same restaurant. The waitress Carol Connelly (Helen Hunt) is the only one who is willing and able to serve him. Divorced Carol lives with her son Spencer, who suffers from severe asthma, which makes many men who are interested in Carol retreat. One day, Simon's house was robbed and Simon was seriously injured. Before being hospitalized, he entrusted the puppy to Melvin for care. Melvin gradually fell in love with this little guy and his psychology changed. He began to pay attention to the world around him, took the initiative to help others, and seriously examined his affection for Carol...