All the Light We Cannot See follows the lives of Marie-Laure LeBlanc and Werner Pfennig during a period of time surrounding World War II.
Marie-Laure is a young girl who lives with her father in Paris. Her father is a locksmith for the Museum of Natural History. Marie-Laure goes blind because of cataracts in her eyes. Her father kindly helps her deal with her blindness by giving her Jules Verne novels in Braille and by creating a wooden replica of their neighborhood so that she can learn to navigate the streets of Paris despite her blindness.
The rumors of the approaching German occupation grow and the museum gives Marie-Laure's father a priceless diamond named the Sea of Flames. He and his daughter evacuate Paris in order to take the stone to a friend of the museum.
The second main character, Werner Pfennig, grows up in Zollverein, Germany with his sister, Jutta. They live in an orphanage that is run by a woman called Frau Elena. Werner is very intelligent, curious, and gifted when it comes to science and engineering. He finds a broken radio and manages to fix it so that he and his sister can use it to listen to radio broadcasts about science. Werner dreams of becoming a scientist but has basically been sentenced to work in a coal mine as soon as he turns 15. He gets a chance to escape this fate when he fixes a radio for a Nazi official and is offered a position at a Nazi school in the town of Schulpforta.
When Marie-Laure and her father discover that the man to whom they were supposed to deliver the diamond has fled the country to go to London,they decide to continue to the house of her great-uncle Etienne in Saint-Malo. Her father once again builds her a model of the city and hides the diamond inside the replica of their house. The Germans confiscate all of the radios in Saint-Malo but Etieene manages to keep one hidden in his attic. Marie-Laure's fatheris asked to return to the musum but is arrested enroute and is placed in a prison camp in Germany.
Werner struggles in the environment at Schulpforta where is only friend is a kind gentle boy named Frederick. Frederick is eventually singled out for punishment because he refused to participate in killing a prisoner of war. He is badly beaten and nearly dies. He is sent home but his mind has received permanent damage. Werner's skills are noticed by a teacher who trains Wener in math that can help him locate radio broadcasts.
Madame Manec, Etienne's housekeeper, has organized a group of women that fought for the resistance against the Nazis. She attempted to get Etienne on board but he felt that it was too dangerous. However, after Madame Manec's death, Etienne and Marie-Laure feel impressed to continue her efforts of resistance. Their method involved Marie-Laure going to a bakery in town where she gets loaves of bread from Madame Ruelle that have slips of paper with Allied intelligence baked on the inside. The information was then delivered to Etienne who would broadcast them on his radio. Etienne eventually gets arrested because he attempts to broadcast the locations of Nazi anti-aircraft guns.
As the war continues, his teacher lies and says that Werner is 18 instead of 16 so that he is set into the military. He joins a special team that hunts down anti-German radio broadcasts and kills the broadcasters. His team moves to Saint-Malo to try and locate another illegal broadcast. Werner realizes that the broadcast seems very similiar to the science broadcasts he listened to in his youth. Werner doesn't tell his team about the broadcast. He locates it himself, finds Marie-Laure and then falls in love with her.
Saint-Malo is bombed by the Allies. Meanwhile Werner takes shelter in a hotel; the hotel collapses under the bombing which traps Werner and his team leader. Marie-Laure hides in Etienne's cellar until the bombing is over. She goes to the third floor to find water but ends up hiding in the attic while a German officer desperately searches for the hidden diamond.
After spending days in the attic, Marie-Laure begins broadcasting on her uncle's radio. Werner manages to fix his own radio ahe and Volkheimer, his team leader hear her reading. While reading she pauses and says that "He is here." Werner understands that she is in danger. Instead of waiting to be discovered, Marie-Laure starts to play loud music. Volkheimer hears the music through his radio and decides to start blasting through the rubble where he and Werner are trapped.Werner manages to get out and rescues Marie-Laure by killing von Rumpel. They escape the city and put the Sea of Flames and the model of the house in an ocean grotto. Marie-Laure reunites with Etienne and Werner is taken prisoner by the Allies. He becomes ill and deliriously wanders into a minefield and is killed by an explosion.
Werner's sister Jutta is in Berlin when the Russian soldiers arrive and some of them raper her.Marie-Laure and Etiennes move to Paris.
Years later, Werner's belongings are given to Volkheimer who gives them to Jutta. One of the items was the model of Etienne's house. Jutta goes to Saint-Malo to find more information about her brother's final days. She realizes that there is a house that matches the model. Someone puts her in touch with Marie-Laure who now works at the Museum of Natural History. Jutta and Marie-Laure meThe et. Jutta tells her that Werner died and gives her the model house. Inside she finds the key to a gate that protects the grotto. She wonders what he did with the Sea of Flames. The narrator reveals that Werner left the diamond in the grotto.