All the Light We Cannot See is told in almost 200 short chapters, and constantly cuts back and forth between two main characters: Werner Pfennig, a young German boy with an aptitude for radio engineering, and Marie-Laure LeBlanc, a young, blind French girl who excels at reading and studying science. The story is told un-chronologically, but the timeline is simplified for the purposes of this summary.
In 1934, Marie-Laure is a 6-year-old with a loving father, Daniel LeBlanc, who works in the Museum of Natural History in Paris. Marie-Laure begins to lose her eyesight, and goes blind. Adapting to this, Daniel teaches Marie-Laure Braille and makes beautiful models of the city of Paris, training her until she’s gradually ready to navigate her way around the actual city. On each of Marie-Laure’s birthdays, Daniel gives her a small “puzzle-box.” Marie-Laure becomes adept at solving these puzzles. As the years go on, Marie-Laure also learns of a diamond called the Sea of Flames that’s kept at the museum. The diamond is rumored to bring eternal life to its owner, but also to kill the owner’s loved ones. Marie-Laure becomes concerned that her father will die from the curse, but Daniel assures her the curse is a myth.
In 1934, Werner Pfennig is an 8-year-old boy who lives with his sister, Jutta, at an orphanage in Germany. Werner is an intelligent boy, though he fears that he’ll be sent to work in the mines when he turns fifteen. The head of the orphanage, a Frenchwoman named Frau Elena, teaches him to speak French as well as German, and encourages him to explore his interests in science. One day, Werner stumbles upon a radio. He and Jutta experiment with the radio, and are amazed to discover a broadcast coming from miles away. The broadcast consists of a series of fascinating lectures on science. Werner develops a reputation for being a great repairman. One day, he repairs the radio of a powerful official named Herr Siedler, who shows his gratitude by recommending him to a prestigious Nazi school, the National Institute. Werner passes his exams with flying colors. Around this time, Jutta begins to grow more distant from Werner.
In 1940, the Germans invade Paris, and Marie-Laure and her father decide to leave the city. Daniel LeBlanc has been tasked with hiding the Sea of Flames from the Nazi invaders. He and three other employees have been given diamonds (three of which are fake, one of which is real) and sent to different parts of the country. Daniel decides to travel to the coastal town of Saint-Malo, to live with his uncle, Etienne LeBlanc. Marie-Laure discovers that Etienne is an eccentric but highly likeable man. Although he’s extremely reclusive, he charms Marie-Laure with his vast knowledge of science and radio.
Werner begins his time as a student at the National Institute, where he’s taught Nazi ideology. Werner wins the favor of Dr. Hauptmann, a professor who gives Werner challenging physics assignments. Before long, Werner is working with Hauptmann to design a complex radar system that will one day be used to find enemy soldiers. Werner also befriends a thoughtful, gentle student named Frederick. Werner learns that Frederick is only in the National Institute because his parents are rich.
In Saint-Malo, Daniel builds Marie-Laure a model of the city so that she can train herself to walk through the streets. Marie-Laure also befriends Etienne’s cook and maid, Madame Manec. She eventually learns that Etienne’s brother, Henri LeBlanc—her own grandfather—made a series of radio broadcasts on science (the same broadcasts that, we realize, entertained Werner and Jutta as children) from the secret radio room in the attic of the house.
One day, Daniel tells Marie-Laure that he has to leave immediately. He promises her that he’ll be back, but after months, Daniel still hasn’t returned. Occasionally, he sends Marie-Laure a letter, in which he claims that he’s in a “good place.” Meanwhile, the German presence in Saint-Malo becomes intolerable—food grows scarce, and the German soldiers arrest innocent people. Madame Manec and her elderly friends work together to undermine the Nazis in small ways. Also, an old man named Harold Bazin gives Marie-Laure the key to a secret hiding place—a grotto in the city wall, at the edge of the sea.
Madame Manec grows sick and dies. Inspired by her bravery, Etienne and Marie-Laure decide to fight the Germans together. Etienne launches a series of secret radio broadcasts, and every day, Marie-Laure buys a loaf of bread from the bakery, which contains a scroll with important resistance information.
At the National Institute, Werner begins to resent his teachers while also enjoying their sadistic games. One day, the teachers order all the students to torture a prisoner. Werner obliges, but Frederick refuses. Soon afterwards, Frederick is savagely beaten—it’s not clear if the students or the teachers are responsible—and he loses most of his mental faculties. Werner loses Dr. Hauptmann’s favor when he asks leaving the school—as punishment, Dr. Hauptmann tells the army that Werner is old enough for military service, and Werner is shipped off to fight. During his time in the army, Werner uses radios to track down enemy broadcasters in Russia, and his fellow soldiers then murder them.
It is now the early 1940s, and a greedy Nazi official named Sergeant Major Reinhold von Rumpel tries to track down the Sea of Flames diamond. He determines that the real diamond is probably in Saint-Malo, where Daniel LeBlanc’s family is staying. Von Rumpel arrives in Saint-Malo around the same time that Werner and his fellow troops are shipped there. Von Rumpel learns that Marie-Laure will know where the diamond is. At the same time, Werner is ordered to find the location of a resistance broadcast network. He determines that the network is located in Etienne’s house, but when he realizes that it’s the same broadcast he loved as a child, he decides to let it continue.
In 1944, the Allies prepare to bomb Saint-Malo. German soldiers, including Werner and his superior, Frank Volkheimer, prepare for an air raid by hiding beneath a hotel. Etienne is arrested as a resistance fighter and sent to jail, leaving Marie-Laure alone in his house. In August, American airplanes drop leaflets ordering all French citizens to leave the town. Marie-Laure, blind, is unable to understand, and stays behind. Late at night, the airplanes bomb the city. Marie-Laure realizes that her father has been hiding something in the model of Saint-Malo: inside the model of Etienne’s house, she finds the Sea of Flames. In the bombing, Marie-Laure is able to survive, but Werner and Volkheimer are trapped in a cellar under the hotel with only a radio.
The Allies continue bombing Saint-Malo. While Werner and Volkheimer try to find a way out of the cellar, von Rumpel goes to Etienne’s house in search of the diamond. Afraid, Marie-Laure goes to hide in the secret radio room. Von Rumpel searches the house but finds nothing—Marie-Laure has taken the diamond with her. In the radio room, Marie-Laure makes her own broadcasts, in which she reads aloud from her favorite book, 20,000 Leagues Under the Sea. Werner hears the broadcast on his radio. He also hears Marie-Laure say that the man in her house is trying kill her.
Volkheimer and Werner decide to use a grenade to bomb their way out of the cellar—amazingly, the plan works. Werner then goes to Etienne’s house to save Marie-Laure. Inside, von Rumpel tries to shoot Werner (wrongly assuming that he’s trying to get the diamond for himself), but Werner manages to overpower and kill von Rumpel. He then calls for Marie-Laure, saying that he’s been listening to her broadcast. Marie-Laure decides to trust Werner, and climbs out. Werner, recognizing that Marie-Laure is blind, leads her to safety. In gratitude, Marie-Laure hides the diamond in Bazin’s grotto, and gives Werner the model house with the key to the grotto hidden inside (Werner has no idea that the diamond exists).
In the following weeks, American soldiers arrest Werner, and Marie-Laure is reunited with Etienne. Marie-Laure and Etienne move back to Paris, and over the next ten years they remain close. After Etienne dies, Marie-Laure becomes a noted scientist specializing in the study of mollusks and whelks. Werner is not so lucky: he’s sent to prison, and dies when he inadvertently steps on a landmine planted by the German troops a few weeks earlier.
In the 1970s, Frank Volkheimer tracks down Jutta, now married with a young child. Volkheimer gives Jutta the tiny model house, in which Jutta finds the key to the grotto. Jutta then finds Marie-Laure, now a middle-aged scientist with a daughter, and gives her the model house. We learn that before Etienne died, he hired a private investigator to determine what happened to Daniel: as it turns out, Daniel was arrested and died of influenza while he was in prison. It also becomes clear that Marie-Laure has left the Sea of Flames in the grotto.
In 2014, Marie-Laure is an old woman with a distinguished career behind her. One day, she goes walking through Paris with her grandson, Michel. She thinks about the people in her life who have become “spirits”: Etienne, Manec, and above all, her father.