In the White Cathedral underground, Alina Starkov lies broken and powerless after her battle with the Darkling, while Mal is kept from her by the Apparat's armed guards. On the surface, the Darkling rules with his shadow army while an outlaw called the Prince of the Air - rumoured to be a Lantsov prince - strikes at the northern border.
Alina maintains a daily deception for the faithful, using Inferni fire and mirrors to fake her lost Sun Summoner power. The Apparat keeps her isolated underground, but she observes Mal training the Soldat Sol alongside Tolya, Tamar, Zoya, and others. In the Kettle, she and Genya apply healing salve while secretly waiting for the master flue to open - their only path to sunlight - and David passes her a hidden note reading "today."
The Apparat confronts Alina in the Kettle, accusing her Grisha allies of a conspiracy to destroy the White Cathedral. Tolya and Tamar appear to have betrayed the group, but Alina realises it is all a ruse - Mal's plan to get the blasting powder delivered and blow open the master flue. Alina summons sunlight for the first time in months, regains her power, brands a Priestguard named Vladim with her palm, and forces the Apparat to submit.
Alina leads a triumphant service, issues decrees to the Apparat, and gathers her Grisha to plan their escape. The group decides to head for Ryevost to seek Nikolai's smuggling contacts, with everyone volunteering to come - including Harshaw and his cat Oncat. After dismissing the Apparat to his chambers, Alina uses her connection to the Darkling to visit him in a vision at the Grand Palace, taunting him before severing the link.
The group journeys through the tunnels, encountering wonders and dangers including an underground river crossing. Mal sets a punishing pace while Alina reflects on Morozova's journals and the firebird. Tension builds between Alina and Mal as he keeps his distance, and the chapter ends with the first explosion of a bombing attack on the tunnels.
The Darkling's bombs collapse the tunnels around the group, separating them. Zoya uses an acoustic trick to locate Sergei and Stigg buried below, and they dig them out. The group emerges from an underground crypt near Ryevost and camps in the woods. Tamar scouts the smuggling post in town but finds no sign of Nikolai's men, and reports that West Ravka has declared for Nikolai. They are captured by a militia led by Luchenko, but are rescued when Nikolai appears and shoots their captor.
Nikolai's forces battle Luchenko's militia, and the group escapes on the Pelican - a prototype cargo airship. Mal, Tolya, and Tamar leap back down to eliminate witnesses, and Alina uses the Cut for her first kill to save Mal. They fly north into Fjerda to Nikolai's secret base, the Spinning Wheel - a hollowed-out mountaintop monastery equipped with hangars, heated floors, and a terrifying iron elevator.
Sergei lets Genya's real name slip and the former King demands her head. In Nikolai's war room Genya faces her abuser, calmly describing the alkaloid poison she designed and laced onto her own skin so that every assault on her would sicken him. Nikolai forces his father to sign a letter of abdication or stand trial for rape, exiling the King and Queen to the Southern Colonies; the King spits back that Nikolai is no Lantsov, and Nikolai bows and answers that he can live with that. After the King calls Genya "ruined", David steps forward with his halting "I know metal" speech, telling her that beauty was only armour but what is inside her is steel, and kisses her. Later, alone in Alina's stone-walled room, she and Mal admit aloud what they both know - that Ravka needs a queen and an alliance, not them - and choose to part ways even as they ache for each other.
The morning after the tribunal Alina finds Sergei hiding in one of the boiler rooms beneath the Spinning Wheel, shaking and unmoored by Marie's death; she arranges for Nikolai to send him to a quiet smugglers' way station near Duva, then sits with him in the dark while he weeps. Out on the south terrace Baghra drags her into a lesson on the mountain Cut, demanding she behead a peak miles away; channelling Morozova's odinakovost Alina sends a clean arc of light across the cloud bank and shears the top off a distant mountain to a roar of soldiers. Nikolai joins her there with the Lantsov emerald ring in his pocket, presses it into her hand as a hedge toward West Ravkan alliance and asks her to consider it. Plans for the firebird hunt come together: Mal stays behind at the Spinning Wheel to practice the Bittern's lines, while Tolya travels with Alina and Nikolai on a state-visit detour to West Ravka before joining the others in the Sikurzoi. That night, the emerald clutched in her hand, Alina slips - half on purpose - across the tether into the Darkling's chamber.
Alina visits the Darkling in a vision and learns he plans to enter the Fold using lumiya - an artificial sunlight compound David invented. He reveals his true name, Aleksander, in an intimate moment before threatening to crush West Ravka. Alina wakes David to confirm the threat is real, and they realise the Darkling may have enough of Alina's stolen light-power to activate the lumiya and cross the Fold with his nichevo'ya.
Alina reveals the Darkling's plans and her ability to visit him in visions. Nikolai sends warnings to West Ravka and pivots strategy. Alina presses Baghra for answers about Morozova, and the old woman reveals she is Morozova's daughter - making the Darkling his descendant. Baghra tells how her father used merzost to resurrect her sister after young Baghra killed her with the Cut, and how villagers threw father and daughter into a river in chains. Nikolai nearly proposes to Alina under a meteor shower as Mal watches from afar.
The Darkling attacks the Spinning Wheel, brought there by Sergei's betrayal. The nichevo'ya tear Sergei apart, and the Darkling transforms Nikolai into a winged monster with shadow corruption. Baghra sacrifices herself by leaping from the terrace trailing the nichevo'ya behind her, giving the others time to escape. The group fights to the Bittern and flees, losing Stigg and Adrik's arm in the battle, while Nikolai flies away as a creature of shadow.
The battered group flies the Bittern to the copper mine at Murin while Alina learns to bend light around the ship to make it invisible. Adrik nearly dies but Tolya stabilises him, and both Squallers push past exhaustion to keep them aloft for two days. At the mine, Alina spots a monstrous Nikolai in the woods - bloody but seemingly still aware of who he is. The group mourns around a campfire, sharing memories of Baghra and processing their losses.
The group splits: Tamar's team secures lodgings in Dva Stolba while Alina travels into the Sikurzoi with Mal, Zoya, Harshaw, and Tolya to hunt the firebird. They pass through the bustling border settlements, buy maps, and climb east toward the legendary Cera Huo - the Firefalls. After days of tracking, they fight off Shu raiders with Grisha fire and press deeper into haunted, sacred territory guarded by eerie ashwood groves and piles of bones.
Atop the Cera Huo cliff, the firebird attacks and nearly pushes Alina over the edge. When Mal catches her wrist, a jolt of power surges between them and both realise the devastating truth: Mal is the third amplifier - Morozova's living legacy passed down through his resurrected daughter's bloodline. Claiming the amplifier would require Mal's death. That night, Mal holds Alina and tells her there is no other way.
Word reaches Dva Stolba that the Darkling has marched on Keramzin and put the orphanage to the torch. Alina hurls herself across the tether and finds him waiting in the charred ruin of the sitting room; he leads her out to the great oak and shows her what he has done. The three Grisha instructors hang side by side in their coloured kefta. Botkin hangs beside them, bloodied from the fight he gave before they strung him up. And next to Botkin sways Ana Kuya in her black dress, her heavy key ring still at her waist - the matron from Keramzin who had been the closest thing Alina ever had to a mother. The Darkling holds her wrists while she breaks, then names his terms: surrender in five days with Mal, or he will march the twenty-three captured students one by one into the Fold. Back in her body, the women of her circle wash her face and comb her hair without a word of blame, and Mal forces her to promise she will use his life if it comes to it. Over juniper smoke at the tavern David and Genya lay out a different plan - bend her light to make the strike team invisible, ambush the glass skiff on the Fold, free the children and put the Cut through the Darkling - and the group agrees that if the ambush fails, the amplifier still binds them to Mal's death.
The group flies to West Ravka, crosses the Fold, and meets the Apparat's Soldat Sol forces at Caryeva. Alina practises cloaking soldiers in invisibility while David and Genya prepare lumiya and blasting powders at an abandoned apple farm. Nikolai appears again in monstrous form - Alina tries to heal him with light but the darkness fights back, and he flies into the Fold in despair. The group prepares for their final assault.
The invisible army enters the Fold in silence and ambushes the Darkling's glass skiff. Their sharpshooters thin his ranks, but reinforcement skiffs arrive and Alina is shot. The plan unravels as the Darkling captures her, but Nikolai appears in monster form to fight the nichevo'ya. Mal finds Alina in the dark and forces her hand around the knife - she stabs him, unleashing Morozova's power. The amplifiers' energy disperses into hundreds of new Sun Summoners who begin destroying the Fold. Alina's own power vanishes completely, but she kills the Darkling with his own shadow-wrapped blade. As the Fold disintegrates, Tolya and Tamar miraculously revive Mal.
Hidden in the barn at Tomikyana, Alina and Mal recover while Nikolai - restored to himself, marked only by faint black lines along his fingers - is smuggled back into Kribirsk and walks in as the prince who broke the Darkling. Quietly, she and Mal absorb what their survival has cost: her power gone, his tracker's sense gone too, the merzost-borrowed second life that made him Morozova's amplifier now spent. Disguised in peasant clothes and Genya's hematite-rust hair dye, they cross the dead sands of the vanished Fold to Kribirsk. In the royal barracks Nikolai offers Alina a place at court and a crown one last time; she sets the Lantsov emerald on his knee, but he picks it up and presses it back into her hand, telling her to keep it and build something new - a parting gift between friends, not a betrothal. She tells Genya she has a pardon, and the job of leading the new Second Army alongside David and Zoya. At dusk the pyre is lit on the sands: Ruby's body, reshaped by Genya into Alina's likeness and crowned with Morozova's antlers, lies beside the Darkling in his black kefta. The Apparat raises his hands, the Inferni strike their flints, the birches catch, and as the crowd cries Sankta Alina she whispers Aleksander once, beneath her breath.
Alina and Mal marry under false names in a small seaside chapel, witnessed only by Misha and Oncat. They rebuild Keramzin as a home for orphans - warm, generous, and nothing like the cold institution they grew up in - living ordinary, quiet lives together at last.