In occupied France, a French Resistance fighter named Fontaine is captured by the Nazis and imprisoned at Fort Montluc. Determined to escape, he studies his cell meticulously and begins to devise a plan, using the limited resources available to him. Fontaine's every move is fraught with tension as he carefully tests the boundaries of his captivity, all the while enduring harsh interrogations and the constant threat of execution. Despite the severe restrictions, he manages to establish communication with other prisoners, garnering invaluable information and support. His cellmate, an older man unable to bear the physical demands of escape, reluctantly consents to Fontaine’s risky endeavor.

As days turn into weeks, Fontaine's situation grows increasingly precarious. Guards increase their surveillance, heightening the tension and threat of discovery. New prisoners arrive, adding both complications and potential allies to his daring scheme. Fontaine's meticulous efforts and cautious optimism drive the narrative as he faces numerous obstacles and makes vital, split-second decisions. The film draws the viewer into his world of anticipation and dread, underlining the fragility of human hope and the courage required to seize a fleeting opportunity for freedom.