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Four prison inmates (Michel Constantin, Raymond Meunier, Jean Keraudy, Philippe Leroy) have been hatching a plan to literally dig out of jail when another prisoner, Claude Gaspard (Marc Michel), 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. Wikipedia