The Devils

Alex is a young thief in the Holy City, burdened by debts and accustomed to surviving through deception, when Duke Michael of Nicaea intervenes during a confrontation with debt collectors and claims that she is his long-lost niece, Princess Alexia Pyrogennetos, daughter of the murdered Irene and rightful heir to the Serpent Throne of Troy. A matching half-coin, a copied birthmark, an Oracle ceremony, and a papal proclamation make the claim politically useful. Pope Benedicta orders the secret Chapel of Holy Expediency to escort Alex to Troy: Jakob of Thorn, an immortal former crusader; Vigga, a Norse werewolf; Balthazar Sham Ivam Draxi, a necromancer; Baptiste, a former pirate and habitual jack-of-all-trades; Baron Rikard, an ancient vampire; Sunny, an elf who can pass unnoticed; and Brother Diaz, the reluctant priest placed in charge of them.

The journey repeatedly tests both the claim and the people protecting it. Duke Marcian attacks the party at the Rolling Bear and is killed by Vigga. In Venice, the chapel retrieves a magical box from an illusionist's trapped house while Balthazar unsuccessfully tries to break Benedicta's binding. Duke Constans later intercepts their ship with soldiers and altered creatures created from Empress Eudoxia's experiments; Jakob kills Constans by exploiting his own inability to die, but the battle wrecks both vessels and scatters the group along the Adriatic coast. While they struggle through a region divided by war, Duke Sabbas hunts Alex with the Dane and other killers. The separated members eventually reunite, Sabbas is killed during the fighting at the Monastery of Saint Sebastian, and the group reaches Troy with three of Eudoxia's four sons dead.

Troy replaces the dangers of the road with a struggle over legitimacy and power. Brother Diaz uses the Athenaeum's archives to defeat many of the nobles' demands, while Duke Michael and Cardinal Zizka promote a marriage between Alex and Eudoxia's surviving son Arcadius, whose fleet and political support could stabilize the throne. Privately, Alex tells Sunny the truth: she is not Irene's daughter. Her mother sold cheese, her father later sold her into thieving, and she knew the real Alexia in the Holy City. After the real girl died of the Long Pox, Alex stole her half-coin, burned a copy of the birthmark behind her own ear, and eventually accepted the identity that Michael and the Church wanted to believe. Sunny urges her not to run away but to use the position she has reached to do some good. Alex is crowned and marries Arcadius, with whom she begins negotiating a workable political marriage.

The apparent settlement collapses that same night. Alex's handmaidens reveal themselves as surviving students of Eudoxia and murder Arcadius. Balthazar discovers that Eudoxia's final experiment succeeded: she transferred her soul into Lady Severa's body, while Severa died in Eudoxia's failing body. A letter from Zizka then exposes a separate conspiracy with Duke Michael. Michael admits that he poisoned Irene, blamed Eudoxia, circulated copies of Alex's papal proclamation so Eudoxia's sons would pursue her, and used Alex as a decoy while he prepared to seize the throne himself. The members of the chapel, already being compelled by Benedicta's binding to return to the Holy City, see Alex's warning from Saint Natalia's Flame and choose to turn back. Rikard, who reveals that the soul-binding never affected him, helps Alex and Sunny defeat Eudoxia's apprentices. Balthazar fights Eudoxia to a standstill and refuses her offer of an alliance. Jakob pursues Michael to the top of the Pharos and tackles him over the broken parapet. Outside the Athenaeum, however, Vigga loses control after escaping one of Eudoxia's grotesque experiments and kills Baptiste.

Alex remains Empress, but victory does not free her or her companions from political obligations. Zizka refuses Alex's request to release Sunny, Vigga, Rikard, and Balthazar from papal control, and Alex agrees to work toward reconciliation of the Eastern and Western Churches in exchange for papal support. Brother Diaz joins the Eastern Church as Father Diaz and stays in Troy as Alex's chaplain. Jakob survives his fall, declines Alex's offer to command a future war against the elves, and returns to the chapel after accepting an icon of Saint Stephen. Sunny and Alex part in the gardens, where Sunny tells Alex that what matters now is what she does next. The bound members depart for the Holy City, with Vigga imprisoned by guilt over Baptiste's death and Balthazar already considering whether Eudoxia's work might help him escape the binding. The final chapter introduces Mother Beckert and Caruso, both summoned to the Celestial Palace as replacements connected to the Thirteenth Chapel, showing that the Chapel of Holy Expediency will continue beyond Alex's mission.