Alina Starkov and Malyen Oretsev are on the run from the Darkling when he recaptures them and unleashes his new creation, the nichevo’ya, on Alina. Alina dispels the attack using her amplifier, Morozova’s collar, but not before sustaining a deep wound in her shoulder. Alina and Mal are held captive onboard a whaling ship operated by the pirate Sturmhond, whom the Darkling paid to help him hunt the legendary sea dragon, Rusalye. Just as they net Rusalye, another ship attacks. Sturmhond and two of his crew, Tolya and Tamar, turn against the Darkling and take Alina and Mal onboard the other ship. They have little choice but to agree with Sturmhond’s plan. With Mal’s help, Alina kills Rusalye, and after much soul-searching, Alina uses the scales to create a second amplifier. She experiments with her new power, and while she is nearly overcome by the results, she feels exhilarated and hungry for more. She and Mal examine Sankta Ilya’s illustration in the Istorii Sankt’ya, discovering that the firebird it depicts may be the source of a third amplifier.  

Sturmhond switches ships to evade the Darkling and takes a small crew, along with Mal and Alina, onboard the Hummingbird, a ship built to navigate both water and air. Sturmhond takes Mal and Alina through the Fold, where they engage in battle with the volcra, vicious monsters that reside inside the Fold. The Darkling’s sudden appearance on the ship’s deck distracts Alina, leaving the crew vulnerable, but Sturmhond’s crew prevails. Alina is distressed at how much sympathy she feels for the volcra. When the Hummingbird emerges from the Fold, it crash-lands on a lake near Kribirsk and the crew swims for the shore. There, they are met by a Ravkan battalion. Sturmhond throws off his disguise to reveal that he is the prince of Ravka, Nikolai Lantsov. When he triumphantly introduces the Sun Summoner as his prize, Alina, angry at his deception, punches him in the face. Nikolai meets privately with Mal and Alina, where he proposes solidifying his power over Ravka by taking command of the First Army and marrying Alina. Alina counters by asking for control of the Second Army. Mal does not like their plan, but he acquiesces.  

The group sets out for Os Alta with the battalion, stopping by every village along the way to meet the people. Alina is reminded of the destruction she helped the Darkling wreak on Ravka, and she learns of a new cult forming around Sun Summoner. Just before they reach Os Alta, Alina designates Mal, Tolya, and Tamar as her personal guards. Outside Os Alta, Vasily Lantsov, Nikolai’s brother, waits with the First Army to challenge Nikolai. Nikolai responds by charming the soldiers. Alina is startled at the crowd of pilgrims locked outside of the city. At the Palace, the ill and aging King and Queen greet Nikolai with tears and hugs. Nikolai negotiates for Alina to take command of the Second Army and for Mal to be granted a reprieve from execution. Alina and Mal proceed to the Little Palace. The Grisha are displeased that Alina is now their leader and make serious allegations against her. Infuriated, Alina summons her power, intending to kill their spokesperson, Sergei. At the last minute, she redirects her power and cracks the Golden Dome instead.  

Alina begins her tenure by abolishing the Grisha hierarchy. She visits Baghra, hoping that she will know something about the firebird, but finds her bitter and broken. Alina and Nikolai plan for war and the Grisha train for combat. Meanwhile, David and the Fabrikators work on a set of mirrors intended to amplify Alina’s powers. Alina spends many late nights in the library searching for clues to the firebird’s whereabouts and further information about the great Fabrikator, Ilya Morozova. Mal frequently accompanies Vasily on hunting parties away from Os Alta. On one of the rare occasions Mal is at the Little Palace, he talks Alina into attending a fortune-teller’s ball, where he will join her in disguise. Their plan goes awry when a fight erupts. They escape the ball, but Alina is stranded alone in a nearby garden. There, the Apparat emerges from the shadows to ask Alina to join him and the pilgrims, tempting her with Morozova’s lost journals. At Tamar’s appearance, he disappears. 

Nikolai is initially encouraged to see that Vasily has attempted to help, but he and Alina are appalled at his callous attitude toward the people of Ravka. During a party to celebrate the success of David’s mirrors, Nikolai renews his advances toward Alina. She is tempted, given the growing distance between her and Mal. Frustrated with herself, she seeks out Mal and finds him brawling with Grisha in the stables. Zoya kisses Mal when he wins. Alina tries to reconcile with Mal and confesses that she has been seeing apparitions of the Darkling, but they remain distant. That night, she awakens to the Darkling’s embrace. She leaves the Little Palace and watches the sunrise with the pilgrims. The pilgrims recognize Alina and nearly tear her apart before Tolya and Tamar rescue her. The pilgrims’ shouts give her a clue about the firebird’s location, and Mal agrees to track it. That night, the Palace celebrates Nikolai’s birthday, which Vasily hijacks to announce a brokered treaty with Fjerda. Alina and Nikolai immediately realize that Vasily has unwittingly sold them out to the Darkling by opening a northern passage for him to access Ravka. Nichevo’ya invade the room and slaughter Vasily as Nikolai retreats with the King and Queen.  

Mal and Alina head for the Little Palace, where Alina manages just one amplified Cut before the mirrors are destroyed. The remaining Grisha gather in the war room and plan their attack. They burst forth with grenades and Alina’s light to find Tolya and Tamar leading a pilgrim army. They all reach the Chapel, where Alina is confronted by the Darkling. He appeals to Alina’s thirst for power, her guilt, and her sense of responsibility. Over Mal’s objections, she joins the Darkling, and as she kisses him, she draws on both of their power to summon as many nichevo’ya as possible. Alina’s plan is to use the Darkling’s power against him, and the exertion depletes them both. Mal tears her away as the Chapel collapses. Alina awakens next to Mal in the White Cathedral, wounded but having been rescued by the Grisha and the Apparat. The novel ends lingering on the hopeless picture of a Darkling-ruled Ravka and the Apparat’s fanatical cult, as Alina can no longer summon light.