Felix's (Jack Lemmon) wife has left him and he is contemplating suicide. His friends sense his depression and one of them, Oscar (Walter Matthau), volunteers to take him in until he is fine again. The two of them are like chalk and cheese - Oscar is fun-loving, gregarious and slovenly, Felix is a shy, stay-at-home, obsessive-compulsive neat-freak. Being around Oscar brightens Felix up, but he quickly starts to irritate Oscar.