Having grown up an orphan, Alina has never felt like she fits in anywhere—and that includes with her childhood best friend Mal, on whom she nurses a secret crush. The two are both in the army and are scheduled to cross the Shadow Fold, a desolate strip of sand and total darkness inhabited by monsters called volcra that divides their country, Ravka, in two. But their sandskiff is attacked part of the way through the journey. As Alina tries to protect Mal, a flash of light blazes bright and Alina passes out.
Back at the army camp, Alina is brought before the Darkling, the most powerful of the Grisha (people who can harness the elements). From others’ testimony, Alina learns that the light came from her—she’s a rare Sun Summoner Grisha. With her, the Darkling can destroy the Shadow Fold and reunite Ravka’s two halves. The Darkling immediately sends Alina to the capital, Os Alta, and escorts her part of the way himself. Alina insists on wearing blue kefta, like the other Summoners, rather than black, which only the Darkling wears. She believes a blue kefta will help her fit in. Though it’s nice to have food, a private room, and a friend—a gorgeous Grisha girl named Genya—Alina is miserable. She can’t call her power without help from her ancient and crotchety tutor, Baghra; she’s as weak and mousy as ever; and she still doesn’t feel like she fits in with the Grisha. To make things worse, though Alina has written Mal weekly since arriving at the Little Palace, she hasn’t heard from him. The King’s advisor and a sort of priest, the Apparat, also appears regularly and seems frightening and predatory.
One day, the Darkling suggests getting Alina a powerful amplifier, which will help her access her power and make her even stronger. He explains that the fabled Morozova’s stag isn’t just a fairytale; the herd is real, and the creatures are ancient and powerful. The stag’s antlers would make Alina unstoppable. Alina is thrilled, but Baghra, for no discernible reason, isn’t.
Weeks later, Alina is feeling particularly abandoned by Mal. She attends her lesson with Baghra angry—and with Baghra’s coaxing, Alina realizes that she’s been using all her energy to subsume her power for years, all so she could stay with Mal. But now that Mal has abandoned her, Alina doesn’t have to do this anymore. Alina apologizes to herself for what she’s done and with Baghra’s help, she learns to manipulate light. Suddenly, Alina has an appetite and is strong. She fills out, her skin clears up, and she begins taking an interest in the world and people around her. However, when the Darkling visits during one of her lessons with Baghra, he insists she’s “not enough.” They still need Morozova’s stag, and the Darkling’s trackers are close. On the way back to the palace, the Darkling kisses Alina—and it’s exciting, but confusing.
As spring approaches, all the Grisha can talk about is the upcoming fete, where the Grisha will demonstrate their powers for the King, Queen, and various nobles—as well as drink and dance all night. Even Alina is excited. It’s a bit of a shock when Alina receives her kefta for the fete and sees that it’s black silk: the Darkling’s color. Still, Alina mostly enjoys the party and, during the demonstration, wows the nobles and royal family with her ability to banish the Darkling’s conjured darkness with light. Afterwards, the Darkling drags Alina to a secluded room, where he tells her his trackers are even closer to Morozova’s stag. They kiss and are on the verge of going further when people bang on the door. The Darkling asks to visit Alina’s room later.
When Alina is on her way back to her room, she notices soldiers—and one of them is Mal. She’s thrilled to see him, but he’s angry. He saw her demonstration with the Darkling and is sure they’re sexually involved, and he hates that the Darkling “owns” her. He also reveals that he hasn’t gotten any of her letters. Alina snaps that she doesn’t want to see Mal again and hurries to her room to sob. But after a while, she hears a knock on the door. It’s Baghra. Baghra drags Alina to a servant’s room, where she explains that she’s the Darkling’s mother—and the Darkling has no intention of getting rid of the Fold. Rather, he plans to enslave Alina by putting Morozova’s stag’s antlers on her, which will allow him to use her power to expand the Fold. He wants control of Ravka and its neighboring countries, and he’ll depose the King and rule through terror. Baghra gives Alina clothes and supplies and helps her slip out of the palace grounds. Alina will head for West Ravka, where she’ll take a ship across the True Sea.
Alina travels on foot for days, certain the Darkling will be after her. Things are fine until she reaches a bigger town and one of the Darkling’s guards spots her. As she tries to evade soldiers tracking her through the woods, Mal appears, sends the soldiers the wrong way, and begins to lead Alina into the woods. He explains that they must head north, to where Morozova’s herd is. They decide they must get the antlers for Alina; that’s the only way to defeat the Darkling. The journey is difficult, physically and emotionally. But by the time Alina and Mal finally find the herd, they’ve admitted their love for each other.
As Alina approaches and even touches Morozova’s stag, she realizes she can’t kill the creature. But the Darkling and his minions appear to kill the stag and fasten the antlers around Alina’s neck. The Darkling imprisons Mal and the party returns to a town on the edge of the Fold, where the Darkling is preparing to lead a group of diplomats from various countries into the Fold to demonstrate Alina’s power. From Genya and Ivan, one of the Darkling’s closest allies, Alina discovers that many Grisha see the Darkling’s power grab as the only way to save Ravka from years of deadly war, even if they find his methods somewhat disturbing.
Finally, Alina, the Darkling, Mal, and the group of diplomats enter the Fold, and the Darkling demonstrates how Alina’s power can repel the volcra. But when they reach the far side of the Fold, the Darkling uses his and Alina’s combined powers to extend the Fold, decimating a Ravkan village. When the Darkling then throws Mal over the edge of the sandskiff to leave him for the volcra, Alina feels helpless—she should’ve killed the stag so she’d have control of its antlers. But then, Alina realizes that choosing to show the stag mercy was a powerful choice in and of itself. With this, Alina regains control over her power. She rescues Mal and leaves the Darkling and the diplomats in the dark to deal with the volcra.
In West Ravka, Alina and Mal burn Alina’s kefta and book passage on a ship going over the True Sea. Alina is haunted by nightmares, but she feels better with Mal beside her.