In the dyers’ village of Saint-Marcel, Paris, in 1664, 18-year-old Alouette steals a scrap of scarlet fabric dyed by her father, René Voland. This color is forbidden to her class by law. As a woman, Alouette is denied entry into the dyer’s guild and is instead forced into the grueling labor of a washerwoman, hauling water from the polluted Bièvre river. The villagers shun her, whispering that she is like her mother, Henriette, who vanished 12 years earlier. Alouette secretly studies the river, believing it holds secrets to the dyer’s art that can free her. She shares a deep, quiet bond with her father, a master dyer obsessed with perfecting a legacy color, Voland’s Scarlet. After a discussion of dye techniques with him, Alouette solidifies her private resolve to defy her station, master the craft herself, and leave her own mark on the world.

In the village of Saint-Marcel, Alouette feels like an outsider, observing the bleak lives of her neighbors and desiring a different life . She secretly cultivates a hidden garden plot, growing dye plants like woad and madder among ordinary herbs. This passion connects her to her family’s legacy: She studies the famous red created by her father while repurposing the knowledge of plants taught by her absent mother, Henriette.

Alouette’s father has reluctantly taught her to read and write. She meticulously documents her dyeing experiments and recipes in a stained notebook, comparing her attempts to her father’s masterful work. Villagers view her as odd, but she embraces her solitary pursuit. In her kitchen, she works with a woad dye bath, frustrated with the imprecise nature of old recipes and her repeated failures. Despite the disappointing results, her resolve is unshaken.

At the factory, Alouette works with a senior dyer, Lucienne, on expensive raw silk for the wealthy Isabel Gobelin’s wedding gown. The Gobelins are a powerful family who control the dyers’ guild. Lucienne advises Alouette to accept their difficult circumstances, explaining that spite is a useless emotion against things that cannot be changed. His pragmatic resignation frustrates Alouette.

After her shift, Alouette walks through a town festival and is distressed to overhear that the old limestone quarry on the Butte-aux-Cailles, a place she considers a personal refuge, is being reopened for work. She sees this as another example of powerful masters exploiting the land and its laborers. Her frustration mounts as she observes the newly hired stonecutters celebrating. Her attention is caught by one young, still quarrier standing apart from the others. Feeling a strange connection to him, she slips away into the crowd.

Quarryman Étienne Duchamp descends into a new limestone mine beneath Paris. He works under the command of master mason Antoine Pelletier, who dismisses Étienne’s warning about a dangerous fissure in a tunnel ceiling. Using skills passed down through his family, Étienne cuts a massive block of stone from the cavern wall. After the difficult and dangerous task of moving the stone to the main shaft, Étienne returns home to his grandmother and the two children, Marie and Thomas, of his deceased cousin, Isabelle, who are now his wards. A cave-in previously killed Étienne’s father and destroyed the family’s livelihood in Rouen, so the family moved to Paris. Étienne comforts Thomas, who misses their old life, and feels the heavy burden of providing for the children. Watching them sleep, Étienne reaffirms that they are the reason he continues the perilous work that he knows will eventually kill him.

In Paris in late 1939, Kristof Larsen, a Dutch psychiatric resident doctor, is burdened by the anxiety of the “phony war,” the prolonged delay of land offensives by France and Britain against Germany since the declaration of war leaves Kristof’s homeland vulnerable to German occupation. At a jazz club, he meets Alesander Extebarria, a Basque architecture student. Alesander persuades Kristof to follow him to a hidden door near Val-de-Grâce and leads Kristof down a spiral staircase into a vast, ancient network of tunnels beneath the city. Alesander explains that these are former limestone quarries, a subterranean “sister city” stretching for hundreds of kilometers. He shows Kristof historical engravings left by prisoners and rebels from past conflicts, suggesting that Paris always endures. When Kristof asks why Alesander has shown him, Alesander replies that he felt he could trust him and that Kristof might be able to “use” the place. Kristof follows Alesander deeper into the underground labyrinth.

It is early 1940. Working in Paris’s Hôpital Sainte-Anne, Kristof observes a new superior, Dr. Blaise Claudel, arriving to implement “Emergency Psychiatric Protocols” (29). Dr. Claudel gathers the resident doctors and announces they must prepare for psychiatric casualties from the front. He advocates for a new, controversial treatment, controlled electrical stimulation, presenting it as an efficient tool to treat soldiers and return them to service.

Kristof is disturbed by the proposal, viewing the experimental therapy as a cold, logistical solution for the war effort rather than a method of healing, and he compares it to historical exploitations of patients. He discusses his concerns with his fellow resident, Julian Broussard, who shares Kristof’s apprehension, having witnessed a medical demonstration that he described as “like something out of Frankenstein.” (34), Julian questions whether they are in any position to challenge Claudel and wonders if such extreme measures might become necessary. The two men share their anxieties about the coming war, their work, and their personal lives, resolving to face the uncertainty together.

Twelve-year-old Sasha Brodsky, feeling confined in her family’s small apartment, seeks a quiet place to study in the hallway. She is confronted by the aggressive concierge, Madame Gagnon, but is defended by Kristof, who is their quiet upstairs neighbor. Sasha’s mother, Rachel, invites Kristof to join their family for dinner. During the meal, the Brodskys reveal they are Jewish refugees who fled Warsaw five years earlier, after Rachel’s father was killed. Sasha is intrigued to learn that Kristof studies the mind, as she has a keen interest in the workings of memory. The evening ends warmly, with Sasha’s father, Felix, inviting the doctor back to play Go, a Japanese board game. Both Sasha and her father observe that Kristof seems kind but lonely.

During the spring of the “phony war,” as Paris indulges in a collective denial Kristof recognizes as “ostrich thinking,” he finds escape by exploring the city’s underground tunnels with Alesander. Kristof, who feels he is making up for a too-serious youth, embraces these risky adventures. One night, Alesander leads Kristof into the locked Montparnasse cemetery grounds, then to an old tower, the Tour de la Charité. Inside, a spiral staircase descends to a passageway leading to a catacomb ossuary. Awed by the spectacle of millions of bones from 20 exhumed cemeteries, Kristof thinks of his deceased sister, Annelies, and feels that loss is perpetual but the dead are never truly gone. Alesander states that what people do while they are still alive matters most.

Alouette remembers her childhood. Her mother, Henriette, was a herbalist who suffered a mental health crisis after a series of stillbirths, culminating in the death of her son. She left the family home when Alouette was six and hasn’t returned, leaving Alouette with a suppressed sense of abandonment. 

In the present, Alouette is inspired in her dyeing experiments after observing a tanner use a harsh chemical process. Deciding that “violence” is needed to unlock a better color, she takes a tin box of arsenic from the tanner’s yard. Back in her home workshop, she adds the poison to her dye vat. Despite the toxic fumes making her dizzy, her experiment is a success; A dipped cloth emerges a dazzling, vivid blue. She wonders if the revolutionary new color will hold fast.

In the early morning, Alouette admires the test cloth which still glows with a uniquely deep and brilliant blue. Her father is stunned by the extraordinary color. Alouette reveals the secret of using arsenic as a mordant. While impressed by the result, René is immediately fearful, warning that the guild masters would never accept such a discovery from a girl and would punish her for it. Seizing the opportunity, Alouette proposes that they use her invention to leave town and make their own name, free from the guild’s oppressive control. Torn between ambition and fear, René dismisses her plan as foolish and refuses to discuss it further.

After his workday in the quarry, Étienne lingers to write his name and the names of his loved ones on a stone wall using soot from his lantern. His private ritual is interrupted by voices from a nearby tunnel. He discovers three men hiding a crate marked with the crest of the Gobelin family. Étienne deduces they are smuggling goods for the Gobelins, likely to evade taxes. The injustice enrages him, reminding him of how the quarrying exploited and killed his father. As he overhears the men discuss future shipments, Étienne privately questions whether honest, backbreaking labor is enough for survival, or if harsher compromises are necessary.

Despite her father’s discouragement, Alouette continues to work with pigments in her kitchen workshop. She is aware that the arsenic in her materials is making her ill, causing tingling, dizziness, and a metallic taste, but she accepts this as a necessary cost for her ambition. She walks to a quarry to gather wild woad, but is dismayed to find the natural landscape has been ravaged and destroyed by the mining operation. There, she meets Étienne Duchamp. He explains his history. Alouette voices her sadness over the landscape’s destruction, and the two find common ground in their shared desire to leave their respective situations. Alouette leaves, feeling confused by the interaction. She looks back to see Étienne watching her.

Returning home, Alouette sees a light on at the Gobelin factory. Outside, she encounters her father’s journeyman, Thierry, who expresses concern that her father is overworking himself. Inside, Alouette confronts René and discovers he is experimenting with the arsenic-based dye she created, despite having forbidden her from using it. He explains he was trying to replicate her method to perfect it, claiming it is for their shared future and that he was protecting her by taking the risk himself. He reaffirms Alouette’s earlier proposal that they will leave once the formula is ready, taking it with them in secret. As he speaks, Alouette notices a distinct tremor in his hand. The sight triggers her memory of another dyer who died from chemical exposure at the factory. Alouette is overcome by a sense of dread.