Werner begins his training, where he cannot help being seduced by the energy and enthusiasm of the drills and exercises. His bunkmate is a frail boy named Frederick. A new character is introduced, separate from both Werner and Marie-Laure’s storylines. Major Reinhold von Rumpel is a trained gemologist who worked appraising gems before the war. Now, as various countries are occupied by Nazi forces, he examines and catalogues the various treasures which are being confiscated. Von Rumpel is being sent in pursuit of various famous treasures, but he is especially intrigued by the legend of the Sea of Flames diamond, and wonders if he would be able to locate it.
German forces now occupy the town of Saint-Malo, creating a sense of unease. Marie-Laure grows sick of being stuck in the house and remains unsure of when she and her father will be able to return to Paris. She notices that her father has started making a scale model of the town of Saint-Malo. As Marie-Laure spends more time with Etienne, he reveals more of the secret features of the house to her. He shows her the cellar, where he is prepared to flee if he feels anxious, and he also shows her the garret on the seventh floor of the house. Up there, Etienne keeps recordings of a program he once worked on with his brother, Henri. Together, Henri and Etienne recorded episodes of a program explaining scientific concepts to children. After Henri was killed in the war and Etienne was sent home, he rigged up an elaborate machine that allows him to transmit the recordings of his brother’s voice. Etienne imagines that one day, Henri might respond to the sound of his own voice.
In a class for technical science and engineering skills, Werner attracts attention for the skill he already has at assembling and repairing devices. Dr. Hauptmann calls Werner in for a private meeting, and another boy is present. The other boy, Frank Volkheimer, is very strong and tough. Hauptmann is fascinated by Werner’s scientific and mathematical aptitude and orders him to start working in the laboratory every night, where he will be supervised by Volkheimer. Werner imagines that he might be able to grow up to be a famous and influential scientist. He writes regularly to Jutta, explaining his new role as a scientist-in-training and telling her about the nationalist and militaristic ideology being inculcated into the young boys. Only dreamy, gentle Frederick seems unaware of what is going on.
In autumn 1940, a perfumer in Saint-Malo named Claude is profiting from the German occupation by selling black market goods. He notices Daniel Leblanc making notes and observations about the layout of the town and plans to sell this information to the Germans. Meanwhile, life becomes more and more difficult under German occupation. An order goes out that all citizens of Saint-Malo must relinquish all radio receivers. Obeying orders, Daniel and Madame Manec hand over all the radios from Etienne’s rooms, but they are unaware of the special transmitting machine in the garret, and Marie-Laure does not tell them about it. When Etienne learns that all of the other radios are gone, he and Marie-Laure discuss what to do about the hidden one. They eventually settle on pushing a heavy wardrobe in front of the door leading to the garret, hoping that even if the house is searched, no one will find their way up there.
In Paris, von Rumpel visits the Museum of Natural History. He pressures the museum directors to show him where the Sea of Flames is kept; they initially refuse, but he threatens their children using information he has gathered from surveillance. Eventually, they show him the ingeniously constructed locked box where a blue diamond is housed.
In December 1940, Marie-Laure’s father receives a cryptic telegram recalling him to Paris. For weeks, he has been overcome with anxiety that he might have the real diamond in his possession, and that, if he does, the curse may harm those he loves. By now, he has completed the model of Saint-Malo, and Marie-Laure has begun using it to memorize the layout of the town. Her father reassures her that his trip to Paris will be brief, and he will soon return to her. However, Daniel Leblanc is arrested during his journey and questioned. Because of his notations about the town, the Germans think he may have been trying to destroy some of the buildings they use as strongholds. Without access to any sort of trial or the opportunity to tell anyone what has happened, he is taken to a German prison camp.