Having stolen some compromising documents from a powerful and successful gangster at a party, a man known as Fred (Lambert) escapes from the police and takes refuge in the underground world of the Paris Métro. There he integrates with the dwellers and befriends several colourful characters, some of which are living under the subway to avoid police arrest like him. While the gangster's henchmen look for his trail, Fred develops a romance with the gangster's young trophy wife Helena (Adjani), who had invited Fred to the spoiled party, and is bored with her gilded-caged life.