In the Grand Duchy of Sieuxerr, a tiny state defined by its rare healing magic, five scholars at the Academie Sieuxerr face the Grand Exam, a five-day practical test that determines whether they graduate as healers. Sieuxerr is still reeling from the serrage, a contagious illness that swept through a decade earlier, leaving severe lingering symptoms and decimating the healer workforce.
On the first morning, Moyna Apprice, a gifted but unhappy scholar whose parents moved to Sieuxerr so their children would be born with healing magic, arrives at her exam chamber to find the body of Ysabel Caudotte, the most brilliant scholar in the cohort. Ysabel's skull is dented, her temple matted with dried blood. The four other members of Moyna's exam group react with shock: Estiene Marcelle, the ambitious son of the Hospital's Head Healer; Alys Tixerrant, a wealthy scholar widely considered a weak healer; Clement Savoye, Ysabel's genderfree best friend; and Jehan Fortier, a charming scholar bonded to a scholarship in the Fenehafen city of Mostau. Estiene confirms with diagnostic magic that Ysabel is dead.
Because the exam's mechanisms prevent interruption for the full hour, they must complete their first case with the corpse in the room. Jehan identifies the patient's rare illness, hare fever. Moyna, who also recognized the diagnosis, deliberately stays silent, considering sabotaging the exam to delay her departure from Sieuxerr; her family's contract obliges her to serve the city of Greteford. Gendarmes arrive afterward but dismiss Ysabel's death as suicide or burnout, a lethal form of magical overload.
A messenger delivers a letter to Jehan, signed by Ysabel, containing the case's solution and a lunch invitation. When Ysabel does not appear, the group concludes she arranged the letters before her death, expecting to be alive. They agree to investigate her murder before the proctor geas, a magical binding on exam proceedings, expires in four days.
Each subsequent morning brings a new exam case, and each proves unusually specific: an old ankle fracture requiring Jehan's unpublished sedation technique; a solo healing of vermeil fever demanding immense magical power; and a traitor illness involving stubborn muscle spasms. After each case, another of Ysabel's pre-arranged letters arrives, addressed to a different group member and revealing her advance knowledge of the exam.
Their investigations uncover layers of corruption. Estiene and Alys learn from the Academie's Secretary that a deal between Estiene's father and the Chancellor guaranteed Estiene a high graduation rank, shattering his trust in his own achievements. They also discover that the Academie plans to eliminate one of its two Dean positions, putting Dean of Assessment Mathilde Brodier and Dean Rogier Saige in direct competition. Brodier, who supervised Ysabel's research, had the opportunity to handpick the exam groups.
On Day Three, Alys's attempt at the vermeil healing spirals out of control. Her magic, secretly enormous due to a childhood blood disease that suppressed it for years, loops back into her, and she nearly burns out before Clement intervenes. Ysabel's letter to Alys apologizes for not helping sooner and urges Alys toward extraordinary things.
That afternoon, Estiene is lured by an unsigned note to find Artificer Floris Decroy dead with his throat slit. Estiene picks up the bloodied knife just as officials open the door and discover him over the body. Gendarme Captain Thoury, pleased to have the Marcelle family under suspicion, arrests Estiene but allows him to continue the exam after Estiene's father, Head Healer Jourdain Marcelle, intervenes.
Moyna possesses a unique, secret ability to send diagnostic magic through the dead. She examines both victims and finds identical traces of healing magic concentrated around their hearts, consistent with deliberately disrupted cardiac rhythm. A confirmed burnout victim's body shows an entirely different pattern, proving Ysabel was murdered by another healer. Thoury privately offers Moyna a position as forensic diagnostician, a role that would let her stay in Sieuxerr instead of fulfilling her contract with Greteford.
Meanwhile, Jehan's secret emerges: since first year, he has been blackmailed by Guion Blavot, a criminal who runs an illegal fighting ring, into stealing Hospital supplies and performing unauthorized healing. Estiene discovers this when he follows Jehan one night, and their confrontation exposes years of mutual pain from a secret relationship Estiene ended out of fear. On Lantern Night, the two finally confess their love and reconcile.
That same evening, the group opens Ysabel's largest letter, which contains her complete research thesis. It reveals that the fifth exam case will be to cure the serrage and that each previous case demonstrated one component technique of the combined cure. The thesis's radical final element replaces the traditional hierarchical healing array with a circle in which all healers work in absolute unison, inviting the patient's own magic to participate. The cure currently works only on patients who are themselves healing mages.
Clement reveals they found Ysabel's research notebook locked in Brodier's desk. The group determines the motive: Brodier wanted to claim the technique to secure her endangered position. When Ysabel refused to share credit, Brodier killed her and stole the notebook. She later killed Decroy when the artificer tried to blackmail her, framing Estiene in the process.
On Mathé's Feast Day, Blavot's men abduct Jehan, intending to make him miss the final exam and repeat the year under Blavot's control. Estiene turns to his grandmother Marguerite, who has strong political connections, and persuades Jourdain to spend his remaining favors on a rescue. The price is Estiene's promise to remain in Sieuxerr and work at the Hospital. Marguerite and Jourdain bluff Blavot into releasing Jehan by claiming Jehan was their intelligence agent all along.
The group arranges for their final exam in the grand Teaching Theatre, with sympathetic artificers making their usual chamber unusable. Behind illusion-curtains, students, faculty, and gendarmes fill the seats. Their patient is Perrin, a healer with the serrage. They first perform the healing using the hierarchical array Brodier knows. It fails. Brodier descends and insists they try again, claiming the technique as her own. On the second attempt, Clement leads the group in Ysabel's true technique: the circle array. Alys provides enormous power without losing control, and Perrin's own magic awakens to join the effort. The serrage is cured.
When the illusion-curtains drop, revealing the packed Theatre, Clement publicly attributes the technique to Ysabel. Moyna presents her forensic findings. Ysabel's mother, Adala Caudotte, asks Brodier directly if she killed her daughter. Brodier confesses to both murders, then uses her own magic to destroy her heart irreparably.
The remaining examiner confirms the group has passed. Each member faces a transformed future: Alys accepts a position on the Hospital's interventionist track; Moyna takes Thoury's offer as forensic diagnostician; Estiene stays in Sieuxerr as promised, while Jehan will remain for the summer before departing for Mostau. Clement plans to continue Ysabel's research toward a cure for non-healers. The final letter from Ysabel, addressed to Clement, never arrives; Ysabel likely intended to deliver it in person. At the feast-day concert, Clement and Alys share their first kiss as the Academie song begins, ready to step from gray scholar's robes into the blue of qualified healers.