Having stolen some compromising documents from a powerful and successful entrepreneur/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 stations and tunnels. There he joins the dwellers and befriends several colourful characters, including others who are living under the subway to avoid police arrest. While the gangster's henchmen try to find Fred, he develops a romance with the gangster's young trophy wife, Héléna (Adjani). She had originally invited Fred to the party featured at the opening of the film, and is bored with her gilded-caged life.