A woman named Alex flees her pursuers by jumping on Brother Diaz’s carriage before falling into the gutter. Hurt, she heads into the fish market to escape but is cornered in an alley by men who have come to collect on her debts. Alex tries to pay them back with a fake holy relic, but they mock and beat her.

The men stop when a wealthy man who identifies himself as Duke Michael of Nicaea appears with his servant Eusebius. Duke Michael claims Alex is Princess Alexia Pyrogennetos, the long-lost heir to the Serpent Throne of Troy. Everyone, including Alex, is baffled, but she has the right birthmark, confirming the story. Michael offers to pay the men, then threatens them when they refuse to release Alex.

Brother Diaz arrives at the Celestial Palace only to be informed that he will not be meeting the pope herself, but Cardinal Zizka, the head of the Earthly Curia. Zizka warns Brother Diaz of growing threats against the Church: schisms, corrupt rulers, monsters, and the looming menace of the cannibalistic elves. She explains that he has been chosen to lead the secret 13th Chapel, the Chapel of the Holy Expediency, a branch of the Church dedicated to handling the Church’s most morally questionable but necessary tasks.

She introduces Diaz to Jakob of Thorn, a scarred and intimidating Knight Templar, and Baptiste, a flamboyant lay assistant, who will work with him. A confused and anxious Diaz tries to argue that his skills are in administration, not clandestine operations or theological warfare, but Zizka insists.

Alex stands in a luxurious room overlooking the Holy City, now clean and dressed in fine robes. As she contemplates stealing a silver comb left behind by the nuns who cleaned her up, she’s interrupted by Duke Michael’s arrival.

Duke Michael explains that he’s Alex’s uncle; her mother Irene should have been empress when their mother died, but their sister Eudoxia murdered Irene and usurped the throne. He tells her about the grand city of Troy and that it’s time for her to reclaim her rightful place on the throne. Though she’s suspicious, Alex resolves to make the most of her situation. After Duke Michael leaves, she slips the comb into her sleeve.

Brother Diaz is awed by the grandeur of the Chapel of the Holy Expediency, but Jakob tells him they won’t be spending much time there. He and Baptiste take him down a hidden staircase that leads to the dungeon below. When they descend, Diaz notices the architecture shift from marble walls to the ancient, seamless stone created by the Witch Engineers of Carthage, the empire that once controlled most of Europe.

The trio passes into a prison where Diaz meets “the devils” held there, who will all be a part of his team: Balthazar Sham Ivam Draxi, a magician; Sunny, an elf able to disappear; and Baron Rikard, a withered vampire. The final cell is empty, as its occupant was deemed too dangerous to be kept there. Afterward, Baptiste and Jakob warn Diaz that success in his new role will depend on flexibility, cunning, and restraint.

Alex is led through the Celestial Palace by Cardinal Bock and Duke Michael. While Bock claims there aren’t any doubts about Alex’s identity, official tests still need to be performed. She takes Alex to a magically sealed chamber and tells her to stand in the sacred circle in the center of the room.

A pair of chained and corpse-like Oracles are then brought in. They take Alex’s hands and ramble out prophecies of fire, trials, a tower, a monk, a knight, a painted wolf, and Alex’s title: Pyrogennetos. The test ends with the Oracles screaming, but Cardinal Bock declares the result successful, and the Church recognizes Alex as the rightful heir to Troy.

After the ceremony, Alex is brought to a formal dinner with Cardinal Zizka and Duke Michael. She tries to behave like royalty but fails when she gets too drunk. She also learns she has four cousins, Eudoxia’s sons, all power-hungry dukes with vast resources and access to forbidden sorcery. Eudoxia, now deceased, experimented with magically blending humans and animals to access and control souls. Zizka and Duke Michael tell Alex she will be secretly escorted to Troy, and a papal bull, or document confirming her identity, will be sent to Michael’s ally, Lady Severa.

Balthazar reflects with disdain on his current state as an unwilling member of the devils. Stripped of his dignity, clothes, books, and freedom, he considers his presence among them an insult, as he’s “one of the top three necromancers in Europe” (45).

A group of acolytes arrives in the prison, accompanied by Cardinals Zizka and Bock and the Child Pope, Benedicta the First. While the group sets up, Balthazar steals a blank prayer sheet as part of a possible escape plan. The Pope then applies a “binding” on each of the devils, drawing a red line across their wrists to secure their obedience for the upcoming mission to bring Alex to Troy. Balthazar dismisses its power, but when he begins plotting his escape again, he becomes violently ill.

Alex struggles to ride side-saddle as she travels with her convoy, comprised of Duke Michael, a group of Papal Guards, a maid to attend her, and the devils. She approaches Sunny, curious about the presence of an elf in the group, but is met with sarcasm.

Meanwhile, Balthazar, perched atop a massive wagon with Baron Rikard, tries to remove the Pope’s binding. He draws a crude counter-spell using the stolen prayer-sheet, but his attempt to break the binding fails, and he throws up again.

Before long, Sunny warns Jakob that they’re being followed. The convoy tries to flee to the safety of a nearby inn, but an arrow kills the wagon driver, forcing Balthazar to take the reins. When they arrive in the inn’s courtyard, he crashes the wagon and is thrown through the building’s front wall.

The group scrambles to fortify their defenses inside the inn’s courtyard. Jakob takes command and organizes a defense. As the attackers gather outside, Baptiste reports that they aren’t human but human-animal hybrid experiments created by Eudoxia. As they descend into the courtyard, Duke Michael rushes Alex into the inn. A sorceress, one of Eudoxia’s disciples, blasts her way in after them. She knocks Duke Michael aside and kills the few guards that came with them, Alex’s maid, and the innkeeper.

Outside, Brother Diaz panics and prays for divine intervention as the half-human attackers slaughter the guards. Most scale the walls, but the gate is finally knocked down by a massive goat-headed creature.

Balthazar regains consciousness inside the now-burning inn and sees Alex cornered by the sorceress. Compelled by the pope’s binding, Balthazar throws himself between them, attempting to shield Alex using his improvised spell sheet. As the sorceress prepares to attack again, Balthazar animates the corpses of dead guards to attack. The undead swarm overwhelms her.

Outside, Jakob kills several half-human attackers, only to be hit by arrows from the rabbit-woman archer. He staggers to the wagon, but before he can unlock it, he’s stabbed by Duke Marcian, Eudoxia’s youngest son and the man leading the assault. As Alex stumbles out of the burning building, Marcian taunts her with the official papal document declaring her heritage. She grovels in the mud, begging him to spare her, but he ignores her.

Brother Diaz stumbles between them and begs Marcian to wait. Marcian is distracted long enough for Sunny to unlock the wagon. Its door crashes open, and a massive wolf-like creature emerges, instantly killing a nearby bull-man in a spray of gore.

When the Vigga-Wolf is released from the wagon, she tears through the attackers, leaving a trail of mutilated corpses. Finally, she reaches Duke Marcian, crushing him and throwing his remains over the wall. She then turns on Alex and Brother Diaz, who are cowering in the mud. As she prepares to attack them, Jakob stands back up and shouts at the Vigga-Wolf like she’s a disobedient child. She hesitates, then reverts to her human form: Vigga. She cries and throws up the viscera she consumed.

With the battle over, the devils gather in the courtyard. Baron Rikard steps out of the ruined inn, appearing younger after consuming blood, and notes to a horrified Brother Diaz that the carnage is not unusual for the devils.

After the fight, Alex is dressed in her murdered maid’s clothes to hide her identity. Baptiste digs the arrows from Jakob’s body as he tells Brother Diaz that he was cursed long ago to never die. Balthazar reappears as well, helping the wounded Duke Michael from the ruined building.

As the group plans for their next move, Duke Michael insists that returning to the Holy City is too dangerous—someone there betrayed them. The only safe option for Alex is to finish the journey to Troy and claim her throne. While Brother Diaz tries to convince them to head back anyway, an angry Alex insists on pressing on. Duke Michael, too injured to continue, returns to the Holy City alone. When he and Alex say their farewells, he reminds her that Lady Severa will be waiting for her.