This story is told in a nonlinear fashion and covers the years 1934-2014. For this review, the novel will be summarized according to the actual timeline, rather than how the story is told.
Marie-Laure LeBlanc is 6 years old in 1934 when she first learns of “The Sea of Flames.” This priceless diamond is hidden in the museum where her father works. It is said that the owner of the diamond will live forever, but that anyone the owner loves will fall under its curse. The only way to break the curse is to throw the diamond into the ocean so that the “God of the Sea” can claim it.
Marie’s father is the principal locksmith at the museum. He creates wooden puzzle boxes for her every birthday. She must unlock the box to retrieve her present. When disease claims her eyesight a short time later, he builds her an intricate model of the city so that she might learn every street and building. He teaches her how to navigate the city in real life so that she does not become housebound.
Her father saves to buy her an expensive Braille book for her birthday — Around the World in 80 Days. She is transported by the story and reads it every day. She spends many hours at the museum with her father. Often, she explores the mollusk collection, learning each creature by its size and the way it feels to her fingers.
Eight-year-old Werner Pfenning grows up with his younger sister, Jutta, in an orphanage in Germany. Naturally bright and inquisitive, Werner and his sister learn all that they can from scraps and items they find around their city. It takes Werner only a few days to get a discarded radio to work again. The children at the home are thrilled to listen to various music and theater programs.
Werner and his sister stay up late to listen to an educational program broadcast by a Frenchman. His musings thrill the children and feed their hungry imaginations. Both Jutta and Werner worry when Nazi propaganda starts to take hold in their city. Although Werner dreams of studying in Berlin, he believes his only future lies working in the nearby mines. However, his knack for repairing radios becomes known throughout the town. People pay him with a few marks or extra food for his services. When he fixes the radio of an important German official, Werner is rewarded with a scholarship to a prestigious school.
When Marie turns 11, rumors of war begin to be whispered throughout Paris. She hears stories of German brutality. Her father tries to calm her fears, but soon he is working extra hours at the museum in order to catalog and hide various precious items. They must flee from the city before the Germans invade. They walk several days to a house where her father hopes to drop off an important item. The house, however, has already been abandoned.
They must journey several more days on foot to Saint-Malo, where they find her father’s Uncle Etienne, a wealthy man, mentally scarred by his service in World War I. Unknown to Marie-Laure, the item her father carries is extremely valuable. Three copies of the Flame of the Sea were made. One was left in the museum, but two others, along with the actual diamond, were distributed to three different couriers, including Marie’s father. Nobody knows who carries the true gem as the copies are nearly flawless.
Etienne’s housekeeper, Madam Manec, welcomes the weary refugees into the house. Marie soon comes to love her eccentric uncle. Etienne seldom leaves the house, but he and Marie have pretend adventures around the world. He shows her the ancient radio he and her grandfather built. They used it to send out recordings of her grandfather teaching science lessons for children; the same recordings Werner and his sister once loved. Marie’s father builds her another model city, this one of Saint-Malo. Several months later, he leaves on an errand for work but never returns.
Werner enjoys his classes at school, but loathes the physical training and indoctrination he must incur in order to stay. He becomes friend with a quiet boy named Frederick who enjoys watching birds. Some of the teachers pick on weaker boys, including Frederick. Werner goes along with the hazing so he doesn’t call attention to himself.
Frederick is beaten for his refusal to pour freezing water on a prisoner. Werner is ashamed that he did not stick up for his friend. Other students eventually beat Frederick so severely that he is permanently brain damaged and sent home to live as an invalid for the rest of his life.
Werner’s gift for electronics and math is soon discovered, and he is picked for a special assignment. He learns the formulas to trace radio signals back to their original source. A gentle giant of a boy, Volkheimer, aids Werner and his teacher. Eventually, Volkheimer and Werner are sent off to serve in the army in an elite unit searching for resistance outposts. Werner tracks radio signals. Volkheimer, and the other members of their group, raid and destroy the outposts Werner finds.
Marie-Laure receives a letter from her father in which he explains he was taken prisoner by the Germans. He tells her he is being treated well. Madame Manec and several of her friends become members of the French resistance. Marie often goes with her when she retrieves messages from the baker that Etienne then transmits over the radio.
One day, an old man shows Marie where a hidden grotto lies. He gives her the key to unlock it. Marie is thrilled to discover it is covered with snails and mollusks. She visits it daily on her way home from the bakery. When Madame Manec passes away, Marie retrieves notes from the baker.
Marie receives a letter from her father in which he reminds her of the intricate boxes he used to carve for her birthday. He says she must look “inside Etienne’s house, inside his house,” if she wishes to understand what happened. Marie knows her father is trying to tell her something by repeating the phrase, but cannot figure out his message.
The war is coming to an end. Werner and Volkheimer are on the trail of the French resistance broadcasting somewhere in Saint-Malo. Werner finds Etienne’s frequency, but does not report it as he is reminded of the recordings from his youth. He tracks down the house and follows Marie as she goes to the bakery. He notices another man, a Nazi officer named Von Rumpel, following her. The man asks her about her father.
Marie hides in the grotto and eats the piece of paper the baker has hidden inside the bread. Etienne believes the officer is looking for the transmitter, but Marie realizes he wants the diamond her father must have carried. Her father’s message becomes clear to her. He has hidden the jewel inside the model of Etienne’s house. She finds it there and hides it.
Etienne is taken prisoner, leaving Marie alone in the house. Von Rumpel has been searching for the missing diamond and tracked the last known copy to Saint-Malo — specifically to Marie’s father. Von Rumpel knows the master locksmith must have hidden it somewhere in Etienne’s house. The Allies drop flyers warning the citizens of the city to flee because they are going to be bombed in the evening. Marie cannot read the flyers. She refuses to leave the house, hoping Etienne will return for her. When the bombs hit, it is one of the few buildings to remain standing.
When Von Rumpel enters her house, Marie secludes herself in the hidden room where Etienne keeps his radio. Von Rumpel searches the home for the missing treasure. Marie knows he will kill her if he finds her, but as hours turn to days, she begins to use the radio to broadcast. She reads 20,000 Leagues Under the Sea while Von Rumpel is asleep, as a way to let the townspeople know she is still alive.
Werner, trapped by rubble in the basement of a hotel in the city with Volkheimer, hears Marie’s broadcasts. Werner becomes enamored with Marie’s voice and feels her terror when she says that someone is in her house, directly below her.
After several days without food or water, Volkheimer finally rallies. The two risk using a grenade to blow some of the wreckage out of the way, although it may bring the rest of the hotel down on them. Their plan succeeds, and they manage to crawl out of the debris. Volkheimer has also heard Marie’s broadcasts. He encourages Werner to search for her while he attempts to find food.
Von Rumpel discovers Marie is in the attic. She lays a trap for him, hoping that she might have the courage to stab him when he finds her so that she can escape. As Von Rumpel searches, Werner arrives at the house. The two struggle, and Werner shoots Von Rumpel. He leads Marie out of the house. He tells her he’s heard there will be a ceasefire at noon.
As he guides her along the street, she insists they stop in the grotto. She removes a box from her coat and sets it in the water. Werner gives her a white pillowcase to hold as a surrender flag. He points her in the direction she must go for help, then goes in the opposite, afraid that if Allied soldiers see her with him, they will kill them both.
Before they part, Marie gives him the key to the grotto as a keepsake. She is reunited with Etienne while Werner is turned over to the Americans. He sustained wounds in the bombing and has a reoccurring fever and so is sent to a medical facility. One night in a delirium, he leaves his bed, walks into a minefield and is killed.
Almost 30 years later, Werner’s sister, Jutta, is contacted by Volkheimer. He has been entrusted to deliver her brother’s personal remains. Among them is a small wooden house. Jutta tracks Marie down in Paris to return the model to her. Jutta doesn’t know its significance, other than her brother had it in his coat pocket. Marie unlocks the box, the diamond is no longer inside, but has been replaced with the key to the grotto. It is intimated that Werner returned to the grotto, discarded the diamond to the water, but kept the key and the box as mementos of Marie.
The book ends in 2014. Marie is still alive, still reliving the memories of the war and the people who helped her survive. She wonders if their souls still float through the sky like the radio waves over which she, her uncle and grandfather used to transmit stories to the world.