In May 1944, summoned by the new commander, Werner and his team enter the town of Saint-Malo. They take up residence at what was once a lavish hotel. Von Rumpel, whose health is rapidly declining, receives word that before he was arrested, Daniel Leblanc was staying in Saint-Malo, and von Rumpel decides to go there in pursuit of the diamond. He learns the location of the house where Leblanc had stayed from the perfumer. Through her network of informants, Marie-Laure receives news that Allied forces are expected to arrive in Saint-Malo within weeks. Werner finally catches an illicit broadcast, and he is astonished to realize that the voice is the same one from the childhood program that he and Jutta once listened to. He initially hides the fact that he has detected the broadcast from Volkheimer. Without telling anyone, Werner searches the city until he finds the house which is in the right position to make such a broadcast possible.
Unsure what to do, Werner watches the house, hoping to glimpse the mysterious Frenchman. Instead, he witnesses Marie-Laure leaving the house to go to the bakery. As Marie-Laure is returning, she stops at the secret grotto, where she is approached by von Rumpel, who tries to question her. Terrified that he will find the coded paper hidden in the loaf of bread, Marie-Laure locks herself in the grotto. Von Rumpel continues to stand on the other side of the gate, trapping her there. She swallows the paper with the message in order to keep it secret. Noticing that Marie-Laure has been gone for too long, Etienne goes out to look for her, leaving his house for the first time in more than twenty years. Eventually, von Rumpel gives up in frustration and leaves. Etienne and the baker come and find Marie-Laure hiding in the grotto.
As the weeks pass, Werner continues to hide the illegal broadcasts and dream about the beautiful young girl he saw. After Marie-Laure recovers, she begins to wonder why the German soldier was questioning her. Recalling that her father sometimes hid surprises in model houses when she was a child, she investigates the model of her uncle’s house and finds a gemstone hidden inside of it. Unsure what to do, she keeps it hidden in her pocket. She is also preoccupied because Etienne is going out that night on a mission related to identifying the coordinates of the Germans so that this information can be transmitted to Allied forces. While Etienne is out, he is arrested by von Rumpel. Marie-Laure is worried about her uncle when he does not come, so she refuses the order to take shelter in advance of the bombing.
In August 1944, Werner again hears a young woman reading on the radio. He plays the radio for Volkheimer, explaining that he has known for weeks about this illicit radio and has done nothing to report it. Etienne is imprisoned along with many other Frenchmen with no way of getting word to Marie-Laure. After four days of fruitlessly searching the house, von Rumpel is close to giving up. He learns from another German soldier that the town is being abandoned and that there will be a brief ceasefire the following day to allow for the evacuation of civilians. After four days hiding in the garret, Marie-Laure has run out of food and water. She begins to play the piano record loud enough that it will attract the attention of whoever is in the house. When Werner and Volkheimer hear the music playing over the radio, Volkheimer makes one last desperate attempt at escape by constructing a makeshift barrier and then blasting the blocked rubble with a grenade.
After the grenade blast, Volkheimer and Werner escape. They part ways because Werner is desperate to reach the house from which the girl is broadcasting. He has heard her expressing her fears that she will be killed. Von Rumpel, delirious and confused from his illness, cannot figure out where the sound of music is coming from. Lured by the sound, he begins rifling around in the wardrobe, and as he does, he hears someone else entering the house. It is Werner. Werner and von Rumpel confront each other in the sixth-floor room where the wardrobe is located. Von Rumpel assumes that Werner is also hunting for the diamond and threatens him with his pistol. While von Rumpel is distracted, Werner shoots and kills him.
Hidden upstairs, Marie-Laure hears the shot and then different footsteps in the house. Werner calls out that he has heard her radio, and she comes out. Werner explains that he will help her escape from the town during the ceasefire and tells her about listening to her grandfather’s broadcasts during his childhood. As they wait for the ceasefire, Werner imagines staying with her in the house forever. Neither Marie-Laure nor Werner understands why von Rumpel was searching the house. When the bombing stops, they step out into the street together. Marie-Laure pauses to go to the grotto, where she deposits something where it will be washed away when the tide floods in. She leaves the key to the grotto with Werner, and they part ways. He knows she will be safer if she is not accompanied by a German soldier.