A young farm boy named Dima is sent to close the barn doors during a storm and encounters a terrifying winged creature with black fangs hiding inside. A beautiful Grisha Squaller arrives, subdues the monster with shackles and wind, and leads it away to a waiting coach, where the coachman addresses the creature as "Your Highness."
Zoya escorts the monster-form Nikolai back to Duke Radimov's estate after his escape, maintaining their cover as lovers on a midnight ride. Nikolai's dark affliction - a remnant of the Darkling's power - has been growing worse for six months, and Zoya urges him to marry and secure the throne while he still can.
Nina works undercover in a Fjerdan fish cannery in Elling with Adrik and Leoni, smuggling Grisha refugees out of the country on King Nikolai's orders. Her local contact Hilbrand warns her that the corrupt harbour captain Birgir is on duty as the last of their fugitives board the Verstoten. When Birgir confronts them on the dock and orders his man Casper to signal the other guards, Nina kills Casper with bone shards through the windpipe, then accelerates a cancer already lurking in Birgir's lungs to kill him from the inside, and drives a final shard through the heart of the remaining guard. The Grisha fugitives sail clear, but Nina is left disturbed by how willingly she has come to love the dark power that parem twisted into her.
Nikolai visits Count Kirigin's estate, the Gilded Bog, which conceals a massive underground weapons laboratory where David Kostyk and Nadia are perfecting the izmars'ya submarine fleet. Nikolai collects a vial of the nearly complete parem antidote and orders David and Nadia back to the capital, hinting at plans to stage a fake demonstration of the submersibles for the Kerch.
Nina, Adrik, and Leoni arrive in the Fjerdan mountain town of Gäfvalle, where a recommissioned factory overlooks the valley and the river has been fouled. They take lodging at a nearby convent and encounter a group of novitiate girls secretly riding in stolen soldier uniforms. Nina is drawn by whispered voices of the dead toward the factory, where she senses hundreds of women's graves surrounding the site.
Nikolai convenes the Triumvirate at the Little Palace's war room, revealing that Hiram Schenck and the Kerch Merchant Council know about the izmars'ya and that his monster affliction is worsening. He proposes a six-week international festival under cover of the autumn equinox, allowing them to bring rivals and allies under one roof for intelligence work and a staged submersible failure for the Kerch. The room presses Nikolai on the matter of a queen - Genya, Tamar, and finally Zoya all argue the festival is the ideal moment to bring prospective brides to him, and Nikolai reluctantly agrees to begin the search.
Zoya manages khergud attack reports, confronts the Cult of the Starless Saint gathered outside the capital walls, and accompanies Nikolai as he defuses a potential clash between the Apparat's Priestguard and the Darkling's worshippers. Nikolai invites the cult's young monk leader, Yuri Vedenen, inside for talks, then chains himself in for the night - only to escape in monster form and nearly kill Zoya in a bell tower in Balakirev before Tolya and Tamar subdue him.
Nina and Adrik travel to the river fork to bury Matthias' body in the Fjerdan countryside. Nina digs his grave, delivers a heartfelt eulogy, and finally accepts that his voice in her head was her own all along - the real voices are the dead women calling for justice from the factory mountaintop.
After almost killing Zoya in Balakirev, Nikolai brings monk Yuri before the Triumvirate, where the young zealot reveals that Ravka's miracle sites form a pattern converging on the Shadow Fold. Tolya and Tamar share the legend of the obisbaya - a purification ritual from the ancient thorn wood. Nikolai decides to make a pilgrimage to the Fold, bringing Yuri along to research the ritual that might free him from the demon.
Caught in a blizzard after burying Matthias, Nina is attacked by wolves whose bites carry something like parem. A white wolf with a scarred eye - Matthias' wolf Trassel - saves her from the pack. Hanne, the tall novitiate girl, rides in and drives Trassel away, then reveals she is a Grisha Heartrender when she instinctively slows Nina's heart during their scuffle. They shelter together at a hunting lodge, beginning a wary alliance.
Zoya prepares for the royal pilgrimage, receiving a powerful new sedative from Genya to control Nikolai's nightly transformations and packing reinforced shackles. Their party tours miracle sites across Ravka - the silver-hummingbird seal at Ryevost, the felled forest pointing west toward the Fold - while Nikolai works the crowds and Yuri and Tolya piece together the obisbaya ritual from old religious texts. Their final stop before the Fold is Adena, where a statue of Sankta Lizabeta beside the village cathedral has been split open by a riot of red roses. As Zoya stands at the statue on the cathedral hill, a Shu khergud soldier crashes through the stone veil and seizes her into the sky.
Nikolai fights off two khergud soldiers, and when Zoya falls from the sky the monster rises within him in broad daylight for the first time, giving him wings to catch her. Zoya destroys the remaining khergud with lightning, and Yuri witnesses the transformation. Back at the hive-themed inn the priest has lodged them in, Nikolai confides the truth of his curse to the monk and strategically frames the obisbaya as completing the Darkling's martyrdom, securing Yuri's cooperation for the ritual ahead. Alone with Zoya later in his bedchamber, prompted by her question of why he carries Ravka's weight, Nikolai recalls his boyhood friendship with Dominik - the tutor's whipping boy turned peasant brother, who later died beside him at Halmhend and whose hand he held as he passed.
Nina begins training Hanne to control her Heartrender power under the guise of Zemeni language lessons at the convent, while also working to gain her trust for intelligence on the factory. Back at the stables, Leoni falls ill from testing the poisoned river water, and they learn a novitiate died from exposure to the contaminated stream. Nina discovers Hanne's unhappy history - her father sent her to the convent for being too "unnatural" - and begins teaching her fighting stances alongside Grisha theory.
Nikolai's party reaches Kribirsk, the old gateway to the Shadow Fold, where Zoya is haunted by war memories. They disguise themselves and sail a sandskiff to the Fold, confronting a pilgrim camp of the Cult of the Starless Saint around a massive black stone disk. Zoya's rage at the Darkling's worshippers erupts in a sandstorm, and when it clears, she, Nikolai, and Yuri find themselves transported to a twilight realm with a vast palace of bone-coloured sand - and three monstrous beings bearing down on them.
Nikolai, Zoya, and Yuri battle the monstrous inhabitants of the Fold realm - a dragon, a bee-woman, and a shifting grotesque. The dragon shatters Zoya's amplifier cuff during the fight. The creatures reveal themselves as the Saints - Sankt Juris (the dragon), Sankta Elizaveta (the bee-woman), and Sankt Grigori (the Bodymaker). They offer Nikolai a bargain: endure the obisbaya ritual to be purged of the Darkling's power, and in return, the Saints will be freed from their prison to walk the mortal world.
Isaak Andreyev, a palace guard whom Nikolai befriended during the war, is recruited by Genya, David, and the twins to impersonate the missing king after Nikolai and Zoya vanished from the Fold. Genya tailors Isaak's features to match Nikolai's, and despite his terror and self-doubt, he manages to channel the king's confident swagger just well enough to proceed with the deception.
In the Saints' sand palace, Nikolai learns the true history of Sankt Grigori the Bodymaker, who created the first Corporalki through his experiments with merzost. Elizaveta explains that Nikolai must learn to summon and control the demon voluntarily, using Zoya as bait - when she is threatened, the monster rises to protect her. They begin training Nikolai to face the obisbaya.
Nina infiltrates the factory at night disguised as a Springmaiden, following the voices of the dead through the abandoned eastern wing. She discovers a hidden dormitory ward filled with pregnant Grisha women and girls, all addicted to what appears to be parem, attended by the Wellmother and Springmaidens who administer injections of a ruddy liquid on a regular schedule.