French underground activists are secretly printing and distributing leaflets against the Nazis, and many of them work at a factory that produces German tanks. Their covert sabotage is dangerous work, and the factory becomes a focal point for resistance. When a worker is shot after being caught distributing leaflets, his girlfriend Collette Madeleine Lebeau runs a restaurant and must pretend not to care as two German customers talk about the murder.
A nurse in the hospital, Yvonne Blanchard [Brenda Marshall], and a doctor, Dr. Andre Marbel [George Sanders], emerge as the movement’s leaders. They hold secret meetings at the doctor’s house and in the basement of Collette’s restaurant, weaving a network that stretches beyond the factory into the city’s everyday life. The tension intensifies as the German occupation presses in, and the underground struggles to protect its members from discovery.
Yvonne’s husband, Jean Blanchard [Philip Dorn], a former French soldier, is released from a German prison camp. He has been broken in spirit and initially seeks to cooperate with the Germans to safeguard his family. He also uncovers pamphlets hidden in his wife’s dresser and faces a troubling test of trust when she lies, claiming the pamphlets were for another nurse. He burns them and urges her to reveal the other nurse, complicating their relationship and the wider resistance.
The German commander in charge of the factory decides to move the machines and most of the workers to Germany to curb sabotage. He tasks the doctor with vaccinating the workers, turning public health into a precarious shield for control. The doctor calls an emergency underground meeting at his house, and Yvonne attends, telling Jean she must return to the hospital to participate in an emergency surgery. He follows and discovers she’s headed to the doctor’s house, not the hospital, fueling jealousy and suspicion. When she returns, a tense confrontation leaves her shaken, and she retreats to the hospital.
The next morning, Yvonne assists with vaccinations, and she spots her little brother George Benoit [Raymond Roe] in the vaccination line. That night, George declares he intends to leave France and join De Gaulle’s cause. His parents and Jean try to dissuade him, while a barber, Luigi, becomes entangled in dangerous deceit as he takes money from the Germans as an informer.
Another clandestine meeting takes place in Collette’s basement. Luigi flirts with Collette, attempting to follow her into an anteroom that hides a secret passage to the basement. She signals the group to disperse, and Jean, who had followed her earlier, arrives only to learn that Yvonne had been there. A sense of suspicion grows, and the couple’s trust is tested as the movement’s dangers mount.
That night at home, Yvonne confronts George with the family gun, and she takes it away to prevent him from using it to flee. He protests that he’ll need it for a dangerous escape, but she remains skeptical of any plan that might rely on unknown allies from the Underground. The next day, Jean watches from the window as a Gestapo agent exits Luigi’s barbershop and tails George and his friends. He realizes Luigi is the informer and a traitor who has betrayed the movement.
George and his companions are captured and taken back to the factory, where the workers learn that the Allies have landed in North Africa and have begun a work stoppage. In a heated moment, George delivers an inflammatory speech and is shot by Colonel Pirosh, a ruthless officer who has massed soldiers to quell the protest. Pirosh himself is wounded in the chaos, and the rescue plan hinges on a dangerous promise: if the doctor can operate and save him, the workers will be freed. The operation proceeds, with Nurse Yvonne assisting; at one point she hesitates and hands the doctor the wrong instrument in a moment of tension, but she ultimately provides the correct tool and the operation continues.
Luigi reappears at Collette’s restaurant, still trying to flirt, and Jean follows him there. A confrontation ends with Luigi being knocked through a false door in the closet and down the basement stairs. Jean’s blow reveals the basement as the Underground’s hidden headquarters, and Collette and the others rally to his side. Jean declares that he has taken care of the quisling, and the resistance welcomes him back with a renewed sense of purpose: resistance to fascism belongs to everyone, not just the workers.
The Nazi colonel survives and demands that fifty workers be killed unless the shooter is turned over. The doctor insists on keeping his promise, and Jean chooses to bear the burden rather than sacrifice the others. He leaves a note for Yvonne, confessing that he shot Colonel Pirosh and revealing his plan to turn himself in to save the rest. He also confesses to the doctor and the Underground, showing the gun he used—the same family gun that Yvonne had taken from George.
A British plane is said to be coming that night to extract the Underground’s leaders to safety, with Yvonne among those on board. Yet the truth of the plan becomes complicated when Yvonne goes to the doctor’s house to stop Jean from turning himself in, only to learn that he has already left. She admits to the doctor that she used the family gun, and they discuss the grim reality of Jean’s lung condition, acknowledging that he might not have long to live anyway.
Together they manage a secret transmission to Paris, reaffirming that France is still fighting and that the resistance will endure. Jean, listening from the edge of the Gestapo’s reach, hears the broadcast and finds a fragile moment of hope amid the peril, a testament to courage, loyalty, and the unyielding spirit of those who choose to resist.
