Philippe Gerbier finds himself at the forefront of the French Resistance when he is arrested by the Vichy French police, who suspect him of engaging in Resistance activities. Despite his eventual acquittal due to insufficient evidence, he is sent to an internment camp. Here, he teams up with a young Communist to devise an escape plan, but before they can make their move, Gerbier is transferred to Paris. There, awaiting interrogation by the Gestapo, he seizes the opportunity to kill a guard and make his daring escape.
Upon arriving in Marseille, Gerbier, along with his comrades Félix Lepercq, Guillaume "Le Bison" Vermersch, and Claude "Le Masque" Ullmann, successfully lures Paul Dounat to a rendezvous, uncovering that Dounat is the young agent responsible for Gerbier’s betrayal. Instead of executing their plan with firearms, which they cannot use due to the neighboring house’s new occupants, they resort to strangling their former associate.
In a twist of fate, Félix encounters his long-time friend Jean-François Jardie at a bar and persuades the thrill-seeking ex-pilot to join their underground operations. During a mission in Paris, Jean-François visits his brother Luc, a distinguished philosopher leading a seemingly detached life. Subsequently, he travels to the Mediterranean to assist in evacuating Allied soldiers, along with Gerbier and the shadowy "Big Boss" to London by submarine. In a surprising turn of events, Jean-François doesn’t recognize that the Big Boss is, in fact, Luc, whose true identity remains a closely guarded secret.
Once in London, Gerbier attempts to solicit additional resources for the Resistance from Free French leaders while Luc is honored with a decoration from Charles de Gaulle. However, Gerbier quickly learns of Félix’s capture by the Gestapo and makes a hasty return, parachuting back into the French countryside to confront the situation.
Meanwhile, Mathilde, a Parisian housewife entrenched in the Resistance, takes charge of Gerbier's cell in Lyon. Her skills impress Gerbier, leading him to keep her involved. She crafts a bold plan to rescue Félix, who has been subjected to brutal torture at Gestapo headquarters. Amid this turmoil, Jean-François decides to resign from the Resistance, incriminating himself to ensure he ends up sharing a cell with Félix. Mathilde, Le Masque, and Le Bison pursue a risky rescue attempt in German disguises, armed with a forged transfer order, but their plot unravels when the prison doctor declares Félix unfit for transport due to his severe condition. As a final act, Jean-François offers Félix his lone cyanide pill after witnessing the failure of their mission.
Despite Mathilde's pleas for him to lay low, Gerbier refuses to abandon the fight, asserting that there is no one else capable of stepping into his shoes at this critical juncture. However, he is apprehended by Vichy police and delivered into the hands of the Germans. Facing execution, Gerbier and his comrades learn that if they can reach the far wall of a room before the machine guns cut them down, they may gain a brief reprieve. As the gunfire erupts, Gerbier dashes for safety, just as Mathilde and Le Bison create a diversion by throwing smoke bombs, enabling him to escape.
Gerbier spends a month in hiding within an abandoned farmhouse. One day, Luc arrives, expressing concern for Mathilde, who has been captured and is in danger of revealing critical information due to threats against her daughter. As the tension escalates, Luc hides when Le Masque and Le Bison arrive with news of Mathilde’s release, along with details of two Resistance members being taken prisoner. In a shocking turn, Gerbier orders Mathilde's execution, yet Le Bison staunchly refuses to carry out the order, vowing to save her. It falls to Luc to reason with Le Bison, persuading him that Mathilde would prefer to die rather than betray her comrades.
Together, Luc, Gerbier, Le Bison, and Le Masque venture back to Paris, where they finally locate Mathilde on the street. In a heartbreaking moment, Le Bison shoots her twice before they make their escape. The film concludes with the somber revelation that within less than a year, all four men were captured and faced grim fates, either by their own hands or at the mercy of the Nazis. Ultimately, Gerbier’s fate is chillingly recounted as “on 13 February 1944, he decided not to run this time.”
