When he's released from prison, career criminal Marcel Talon promises his girlfriend Maggy that he'll stay on the straight and narrow from here on in. But he quickly breaks his promise when he joins former accomplices Fred and Smiley to rob a bank in Old Montreal.
The plan consists of digging a tunnel starting from the city sewers to reach a hallway leading to the bank vault. The operation, which takes three months of work, is marked by many tense moments. Vincent, recruited by Smiley, gets on Marcel's nerves and he also has to deal with Annie, his suspicious probation officer.