Set in Nazi Germany in December 1943, this novel follows Franka Gerber, a former nurse and convicted dissident, as she discovers an unconscious parachutist in the snow-covered Black Forest mountains near Freiburg. Franka has walked into the wilderness carrying her dead father's revolver, intending to end her life. Everyone she has loved is gone: her mother to cancer, her brother Fredi to the Nazi euthanasia program, her boyfriend Hans to the guillotine, and her father to an Allied bomb. She stumbles upon a man with a parachute whose papers identify him as Werner Graf, a captain in the Luftwaffe, the German air force. Both legs are broken, and his backpack holds two pistols. Despite her hatred for the regime, Franka decides to save him. She digs a snow cave and shares her body heat with him through the blizzard.
Before dawn, Franka hikes two miles back to her family's isolated cabin for a sled, sets the man's legs with branches, and drags him through the snow. On the trail she narrowly avoids Herr Berkel, the father of her ex-boyfriend Daniel, a captain in the Gestapo, the Nazi secret police.
Over the following days, Franka tends to the stranger, fashioning splints and dressing him in her dead father's pajamas. She pries up floorboards in the spare bedroom, creating a hiding space. She hears him cry out in English in his sleep, confirming he is not German. When he wakes, he calls himself
Hauptmann (Captain) Werner Graf and refuses to explain his mission. Franka offers to report him to the Gestapo; he declines, reinforcing her suspicion that his identity is false.
Franka skis to Freiburg for medical supplies. At a café, Daniel Berkel, now a Gestapo captain, recognizes her. He knows she served time for involvement with the White Rose, a student resistance group, and warns her she remains a convicted enemy of the state. Meanwhile, the man picks his bedroom lock, finds her father's revolver, and confronts Franka at gunpoint when she returns. She argues he cannot survive winter alone without her. He lowers the gun, and she sets plaster casts on his legs.
To earn his trust, Franka tells the man her life story. She describes her teenage years in the League of German Girls, the Nazi youth organization for women, and her relationship with Daniel, then a leader in the Hitler Youth, the boys' counterpart. Her father opened her eyes to the regime's cruelty, and her dying mother urged her not to let Nazi ideology twist her soul. On Kristallnacht in November 1938, when state-sponsored mobs attacked Jewish businesses, Franka watched Daniel lead rioters in beating the local baker until he went still.
Franka describes her brother Fredi, who had a degenerative physical condition and a radiant spirit. Unable to care for Fredi alone, their father placed him in a residential institution. Franka moved to Munich, where she met Hans Scholl, a charismatic student medic who drew her into the White Rose. The group distributed leaflets urging passive resistance; Franka helped mail thousands across Germany. Hans's sister Sophie Scholl joined and became his equal in leading the effort. On February 18, 1943, a university handyman caught Hans and Sophie distributing leaflets; they were tried and guillotined the same day. Franka was arrested weeks later. At her father's urging, she denounced the group, receiving six months in prison while other members were executed. Her father was killed by an Allied bomb during her imprisonment. Upon release, she retreated to the cabin to end her life, the night she found the man.
The man senses Franka is withholding something. She reveals the final piece: Vans belonging to the SS, the Nazi paramilitary organization, arrived at Fredi's institution without warning. Soldiers loaded patients into the vans; one soothed Fredi with promises of ice cream. Days later, the family received a letter stating Fredi had died of a heart attack. Franka blames herself for his death. Moved by her story, the man reveals his true identity: John Lynch, an American agent from Philadelphia with the Office of Strategic Services (OSS), the US intelligence agency.
John explains his mission: He was sent to extract Rudolf Hahn, a German physicist pioneering nuclear fission research who agreed to defect. His plane was hit by flak over the Black Forest, forcing him to bail out far from his drop zone. He asks Franka to travel to Stuttgart in his place. After initial resistance, she agrees.
Over Christmas, Franka and John grow closer and develop a drill for hiding him under the floorboards. Franka obtains crutches for John from her childhood friend Martina Kruger, a nurse in nearby Sankt Peter. After Christmas, Martina, troubled by Franka's request, reports her to the Gestapo.
On January 3, Franka meets Hahn on a park bench in Stuttgart. An Allied bombing raid strikes the city, and Hahn is mortally wounded. As he dies, he reveals that his research concerns a bomb capable of destroying an entire city and directs Franka to microfilm hidden in his apartment. Franka retrieves the microfilm during the chaos. Back at the cabin, John argues the technology will be developed regardless; the only question is who completes it first. Franka reluctantly hands over the microfilm.
Gestapo agent Armin Vogel informs Berkel about Martina's report. On January 21, Berkel arrives at the cabin while John hides under the floorboards. Berkel interrogates Franka, then attacks her. John bursts from hiding with a pistol. When Berkel reaches for his gun, Franka grabs her father's revolver and shoots him twice, killing him.
They hide the body and flee south toward Switzerland, stopping with Franka's great-uncle Hermann, who despises the Nazis. After they leave, Vogel discovers Berkel's corpse and launches a massive manhunt. Vogel interrogates Hermann, who refuses to betray his great-niece; Vogel shoots him.
Near the Swiss border, a truck carrying Vogel and soldiers spots Franka in an open field. John reaches the trees unseen, but a cliff blocks their path. Franka insists John go on alone, arguing the mission matters more than her life. John lowers himself over the edge, and Franka surrenders.
John makes his way to the road, disguises himself in his Luftwaffe uniform, and flags down the truck posing as a downed pilot. He kills the guards and Vogel, though he is shot in the chest. Franka bandages him and drives to the cliff, where she lowers him with rope. She carries his collapsing body toward a Swiss customs office. John tells her to take the microfilm and let him die. She refuses, carries him inside, and forces the customs officer to drive them to a hospital. John survives.
The final chapter jumps to October 1945. Franka has spent months in a Swiss refugee camp near Basel. She reflects on the atomic bombs dropped on Hiroshima and Nagasaki, acknowledging her role in delivering the microfilm that contributed to their creation. Letters from John promised his return, but they dwindled. On her last day at the camp, John appears at the door in full military dress. He tells her he has come back and will never leave her again. She replies that she will never let him, and they embrace.