One morning in early May, Kristof hears a radio broadcast announcing Germany’s invasion of the Netherlands, Belgium, and Luxembourg. The news that his home country has fallen leaves him panicked. Days later, after the Germans bypass the Maginot Line and break through at Sedan, an unnerving quiet falls over Paris.

Kristof receives two letters from his father describing the invasion of the Netherlands, confirming that the country is now at war. The time delay between the events and the news leaves Kristof feeling powerless. At Sainte-Anne’s hospital, his colleague, Julian Broussard, thinks Kristof looks haggard. Kristof vents his frustration over a patient, Henri Allard, who tried to bathe in bleach, and criticizes Dr. Claudel’s intention to use electroshock therapy on such men. Julian, who is trying to start a family with his wife Patrice, sympathizes but admits he cannot afford to challenge authority, as he must protect his family life.

Kristof’s patient, Henri Allard, is a soldier who was left catatonic after the Battle of Sedan, where he was psychologically shattered by the incessant sound of Stuka dive-bombers. Over several days, Kristof works to build trust with Henri, who remains withdrawn but slowly begins to accept small comforts.

Once Henri seems stable enough, Kristof suggests hypnosis. He guides Henri into a peaceful memory from his childhood, visiting his grandparents’ land near Bordeaux. The session is a success, and Henri emerges calmer and more rested. The process of helping Henri find a moment of peace provides Kristof with his own a sense of relief.

The spring season reminds Kristof of March 1935, the year his sister, Annelies, seemingly took her own life. At the time, Kristof was an ambitious medical student focused on a career in surgery. He feels that he overlooked the warning signs of Annelies’s depression. She was brilliant and sensitive, finding meaning in the works of German poets like Hölderlin and Rilke. She left a farewell note quoting Rilke, suggesting that she did end her own life.

Kristof feels that Annelies was, in a way, his first patient. His current work at Sainte-Anne’s, particularly his efforts to connect with traumatized men like Henri Allard, is his attempt to understand the pain he was once powerless to prevent and to atone for feelings of guilt on this.

On the last week of school, Sasha Brodsky senses the rising tension in Paris. She sees new, disoriented refugee families on the streets, which reminds her of when her own family arrived from Poland. She finds a quiet park, the Jardin des Gobelins, and reflects on the Bièvre river, which was long ago buried and now flows unseen beneath the city. She sees the hidden river as a metaphor for the war, which feels both invisible and unstoppably rising. In her notebook, she writes that the war is no longer happening “elsewhere” (82) but is already present in Paris.

In Saint-Marcel, Paris, in 1664, a quarrier named Étienne sees Alouette Voland, a dyer’s daughter, working in her garden. He is captivated by her focused intensity. Her plot contains unusual plants, and he approaches to ask if they are for dyestuffs. Alouette is reserved but confirms they are. When she shifts her basket, Étienne glimpses a cloth dyed a brilliant, deep blue, a color he has never seen before; she notices and quickly conceals it. Intrigued, Étienne senses she has more secret experiments. When he suggests her work must be extraordinary, she offers a rare smile.

During the summer solstice feast of St. John, Alouette is approached by a woman she eventually recognizes as her mother, Henriette, who abandoned her and her father, René, 12 years earlier. Henriette reveals she has been living in nearby Gentilly for a year and has come to warn Alouette. She has heard rumors that the dyer’s guild is suspicious of René’s unsanctioned experiments. Furious at her mother’s sudden reappearance and meddling, Alouette dismisses Henriette and walks away.

Shaken by the encounter, Alouette runs into Étienne near the river. He senses her distress, and she confides in him about her mother’s return and the warning of danger. Finding comfort in his presence, she allows him to kiss her. They are interrupted by festival goers, and he leaves after telling her not to forget to live.

Alouette returns to her cottage and confronts her father, René, with Henriette’s warning about the guild. He dismisses her concerns, calling Henriette a troublemaker. Alouette insists the danger is real and pleads with him to flee Saint-Marcel with their new dye formula. René refuses, arguing that the formula is not yet perfect and that they must be strategic. Alouette counters that waiting is the most dangerous choice they can make. His refusal is final. That night, Alouette lies awake, realizing she may have to escape on her own.

With her father ignoring her warnings, Alouette resolves to leave Saint-Marcel alone. She seeks out Étienne at a rough tavern, Le Lapin Noir, and tells him of her decision to flee to Marseille, where she has family. He walks her home, and on the way, she asks if he would help her if she needed it; he agrees without hesitation. Their mutual attraction leads to another kiss, and when Étienne mentions a room above the tavern, Alouette chooses to go with him, and they spend the night together. Afterward, they share a moment of quiet intimacy. Alouette knows their connection is temporary, but she cherishes the feeling of being understood.

Kristof meets his friend, Alesander, for another exploration of the souterrain, the network of tunnels beneath Paris. This time, Alesander is methodically mapping the passages with a compass and ruler. He tells Kristof that given the escalating war in Europe, the tunnels could soon be needed as hiding places and escape routes, citing his cousin’s experience using secret trails to survive the Spanish Civil War. Though saddened that their adventures have turned serious, Kristof accepts the new reality and helps Alesander map potential escape routes, noting old revolutionary graffiti on the walls.

At Sainte-Anne’s, Kristof witnesses a withdrawn patient, a former soldier named Georges Plomin, being forcibly taken for electroshock therapy under the orders of Dr. Claudel. Kristof follows them and watches in horror as Plomin is strapped down and subjected to a violent, induced seizure. The brutality of the procedure starkly contrasts with the compassionate methods Kristof learned in medical school. After the treatment, Dr. Claudel casually remarks that the patient tolerated it well. The incident solidifies Kristof’s conviction that he can no longer remain a passive observer and must take a stand against such practices.

On the night of June 3, Paris is hit by an air raid. As the residents of Kristof’s building huddle in the cellar, Rachel Brodsky, who is at least a month from her due date, goes into premature labor. With explosions shaking the city, Kristof realizes they cannot reach a hospital. He discovers the baby is in a breech position and, after failing to turn it, is forced to deliver it under dangerous conditions. Just as the all-clear siren sounds, the baby—a girl—is born. She is initially not breathing, but Kristof successfully revives her. The family later takes Rachel and the infant to a hospital. Felix informs Kristof they have named her Klara Miriam, after Rachel’s grandmother, who helped hide Jews during the pogroms.

In 1664, Alouette leaves Étienne’s room at dawn, intending to make one last attempt to convince her father to flee. She arrives at their cottage to find it ransacked and her mother, Henriette, inside. Henriette explains that the guild’s guards came and arrested René for withholding formulas. Deeply mistrustful, Alouette accuses Henriette of betraying her father. Henriette denies it, but Alouette, consumed by anger and fear, draws a knife and forces her to leave. Alone amid the wreckage, Alouette fears for her father’s life and the fate of their work.

Days later, Alouette’s friend Lucienne visits and tells her that the guild is accusing René of treason for trading formulas with Venetian dyers. Alouette is stunned but recalls her father’s recent desperate and secretive behavior. Later, Étienne arrives and confirms the charge. He offers to help her escape Saint-Marcel that night. Though tempted, Alouette cannot abandon her father or the dye formula she created. She refuses Étienne’s offer to flee. Étienne gives her a small stone lark he carved for her; she places it back in his hands, asking him to keep it safe.

Convinced her father has been framed, Alouette believes his private ledger, hidden in his factory workshop, contains proof of his innocence. She breaks into the factory at night to retrieve it. While searching, she is discovered by guards. In her attempt to escape, she runs through the dyeing room, trips on a scaffold, and falls into a vat of scarlet dye. The guards try to capture her as she struggles in the thick liquid. After breaking the surface to breathe, she is seized and knocked unconscious.