In 1944, von Rumpel is just outside of Saint-Malo, watching the bombing. He knows that the Germans are in danger of losing the war, and he is anxious about his own declining health. Still, he is determined to enter Saint-Malo as soon as he can. When there is a pause in the bombing, von Rumpel sets off on foot and makes his way to Etienne’s house. Inside, Marie-Laure is hiding in the cellar with a small supply of food, while Werner and Volkheimer try futilely to dig themselves out of the cellar in the hotel. Werner wants to give up, but Volkheimer encourages him not to lose hope. Eventually, Marie-Laure leaves the cellar in order to get more food and relieve herself. While she is on the third floor of the house, she hears someone entering.
Back in 1940, after Frederick’s beating, Werner is deeply ashamed that he failed to help his friend. Nonetheless, Frederick invites Werner to come home to Berlin with him and meet his family. Frederick comes from a wealthy family, and Werner is shocked by the luxury he witnesses during his visit. When Werner tries to get Frederick to tell him about his hopes and aspirations, Frederick brushes his friend aside, claiming he has accepted his fate and the role he has to play. Shortly after they return to school, Frederick and Werner witness a horrifying spectacle where a prisoner is tied to a stake in the freezing cold while officials and the boys all take turn throwing buckets of water on him. Frederick refuses to participate.
Meanwhile in Saint-Malo, Marie-Laure and Etienne have no idea what has become of her father. They know he never reached Paris but not why. As days pass without word from him, Marie-Laure becomes more and more depressed. Eventually, Madame Manec takes her out of the house after months of being inside, and they go to the seashore. There, Marie-Laure recovers a will to live. Her father also eventually sends a brief note, reassuring her that he is being treated well at a camp in Germany.
In Paris, when he is shown the blue diamond at the museum, von Rumpel experiences momentary triumph but quickly realizes the stone is a fake. He tracks down the man who most likely would have been responsible for creating duplicates of the stone and arranges for him to be arrested. Von Rumpel questions the man and learns that there are three duplicates of the stone. His sense of urgency to find these other stones is heightened by the news that he has been diagnosed with cancer. He is familiar with the legend that whoever possesses the Sea of Flames will live forever, and he becomes obsessed with the idea that obtaining the stone will prevent his impending death.
Frederick’s refusal to participate in torturing the prisoner to death makes him a target for the officers and other boys. He is beaten regularly during drill exercises. Werner tries in vain to protect his friend, but he is also preoccupied since the mathematical experiments he has been working on are being tested and coming to fruition. Werner can now use the transceivers to successfully determine someone’s location, and he chooses to ignore the clear implication that this technology will be used to track and kill enemy soldiers. One day, Frederick is missing. Werner goes to the infirmary and learns that Frederick was seriously hurt after being attacked by the other boys. Frederick has been sent away for surgery but is unlikely to recover. Werner sinks into guilt and shame about the work he is engaging in but has no choice but to continue. Werner knows that Jutta would be disgusted with him. In January 1942, Werner asks permission to visit home but is refused.
Marie-Laure continues to go on daily walks with Madame Manec, exploring the beach and the town of Saint-Malo. Madame Manec always tries to provide charity and help to other residents of the town, including a man named Crazy Harold Bazin, a homeless and disabled veteran of World War I. As she meets Madame Manec’s friends, Marie-Laure learns that many of the women in the town are working together to engage in small acts of resistance against their German occupiers. Madame Manec also tries to encourage Etienne to use his skills to participate in the resistance, but Etienne is too afraid. One day, Harold shows Marie-Laure a secret grotto which is partially flooded when the tide comes in. He gives her the key to access this grotto. A short time later, Harold mysteriously vanishes, and townspeople fear that his disappearance is connected to his participation in the resistance.