Four prison inmates have been hatching a plan to literally dig out of jail when another prisoner, Claude Gaspard, is moved into their cell. They take a risk and share their plan with the newcomer. Over the course of three days, the prisoners and friends break through the concrete floor using a bed post and begin to make their way through the sewer system -- yet their escape is anything but assured.
After the failure of his recent criminal exploits, international con artist Alexandre Dupré sets off for Venice to try to dupe some Japanese business men into buying a fake copy of a stolen Caneletto. On the aeroplane, a stranger asks him to look after his briefcase until after they have landed. Alexandre agrees, but the stranger is shot dead soon after and Alexandre himself becomes the target of enemy agents who are keen to recover the briefcase...
A bus conductor gets dressed for work in the morning, goes to the toilet, where he is killed by a bomb. The Commissioner and his fat, bumbling assistant, Inspector Charbonnier are put on the case. After interviewing friends, wives, colleagues, and spying on strangers who might be connected, our heroes trace the assassin down to a mental institution where, it seems, the murder victim has been an inmate for the last three years...
