Before the start of the film Héléna, the wife of a wealthy man, had invited Fred, who she had just met, to her birthday party on a whim. Fred, who carries
dynamite sticks on him, had blown open a safe in the house and stole some papers. Dressed in a tuxedo from the party, he flees in Héléna's car from her husband's associates, also dressed in tuxedos. When Fred crashes his car into the entrance of a
Paris Métro station, the chase continues on foot and inside a metro train, where Fred manages to lose his pursuers. He then calls Héléna to arrange an exchange of the papers for money. During the exchange, Fred accepts a far smaller amount than arranged and he and Héléna seem to acknowledge that they have feelings for each other. Her husband's associates try to capture Fred, but he escapes by jumping under a train and then entering deeper into the labyrinthic backrooms of the metro, but not before informing Héléna that he has not returned all of the papers. He later calls her and tells her he loves her. In the bowels of the metro Fred meets and befriends several persons: Jean-Louis, a small-time thief who always wears
roller skates and lives in one of the backrooms, where he lets Fred spend the nights; Big Bill, a strongman who works on the tracks and twice helps Fred lose handcuffs; a flower seller who buys Fred's tuxedo; a bass player and a drummer who Fred asks to start a band together with a singer Fred knows. Jean-Louis also suggests robbing a pair of money couriers who use the metro station every week. The next morning, Fred arranges to see Héléna alone in exchange for the documents. Since her husband's associate Jean does not let her meet Fred alone, Héléna requests the help of metro police commissioner Gesberg and his assistant Batman who have tried to catch Jean-Louis for several months. After noticing that the flower vendor wears Fred's tuxedo, Gesberg coerces the flower vendor to tell him that Jean-Louis is helping Fred. Héléna manages to find Jean-Louis by herself and he brings her to Fred. She draws a gun and requests the papers. Fred, who seems amused, tells her that he does not have them and gives her incorrect information which she follows. She later comes back but no longer threatens Fred. Instead they spend the night together in the empty metro station with Jean-Louis, the drummer, Big Bill and others, stealing food, playing music, dancing and smoking
marijuana. The next morning, her husband shows her a ransom note ostensibly by Fred for 50.0000.000 Francs for the documents and takes her back to their home. He also asks Jean to "take care" of Fred. At a party later that evening at a magistrate's home, Héléna defiantly insults everyone and leaves. She comes back to Gesberg and requests him to bring in Fred before her husband's associates kill him. Batman meets Fred by accident but fails to arrest him. Gesberg manages to take in Jean-Louis and finds out it was he who wrote the ransom note. Fred and the flower vendor successfully rob the money couriers and Fred uses the money to arrange a concert in the metro station with the band that Fred had organized. The concert is a success. During the concert, Héléna sees Fred and happily runs toward him. At the same moment, Jean also sees Fred and shoots him. Fred collapses to the floor and a devastated Héléna kisses him when he asks her whether she loves him a little bit. She angrily and violently rebukes Jean when he tries to take her back to her husband, while Fred is seen still lying on the floor but happily singing along with the band. ==Cast==