Three months after escaping from Under the Mountain, Feyre is not living happily ever after. Although she is engaged to Tamlin, she suffers terrible nightmares, and she fears the arrival of Rhysand to enforce their bargain: a week out of every month to be spent at the Night Court. Depressed, Feyre relies on Ianthe, a High Priestess, to take care of the wedding arrangements. Although Feyre longs to help rebuild the Spring Court, Tamlin, fearing for her safety, keeps her trapped at home. On her wedding day, Feyre realizes she can’t go through with the ceremony, and Rhysand interrupts to call in their bargain. At Rhysand’s beautiful palace, he tells her that she might have special powers and should learn to use them. He also coaches her to learn to read and to use her mental shields. Seeking an alliance with Tamlin, Rhysand tells Feyre about an impending war with the King of Hybern, who plans to destroy the wall that protects mortals from faeries. When Feyre returns, Tamlin interrogates her about the Night Court. After he refuses her request to train her powers, she is angry, and their relationship begins to disintegrate. 

After Feyre returns from her second visit to the Night Court, Tamlin puts her under even heavier guard. Feeling trapped, Feyre begs to accompany him on an expedition, but he locks her in the house. Desperate, Feyre cocoons herself in darkness, ice, and fire. Mor rescues her and takes her to the Night Court, where she decides to stay for a while. When Rhysand prepares to leave on a mission, she begs to go with him, and he agrees, taking her to his secret city, Velaris. Feyre learns that Rhysand became Amarantha’s whore to conceal the city from her. In Velaris, Feyre meets Rhysand’s Inner Circle, all misfits with exceptional powers, and agrees to work with him. Her first task is to ask the Bone Carver about the King of Hybern’s plan to resurrect a human warrior called Jurian. The Carver confirms that the King of Hybern will use the Cauldron to perform the resurrection and also to destroy the wall that keeps mortals safe. He reveals that the Cauldron can be nullified by the Book of Breathings, which is divided between the Summer Court and the mortal queens.  

Feyre and Rhysand go to Feyre’s family home to persuade her sisters to host a meeting with the mortal queens. Then, Feyre, Rhysand, and Amren (his second-in-command) use a diplomatic visit to the Summer Court to steal the Book of Breathings from its hiding place in a temple. Finally, the mortal queens agree to meet at Feyre’s family home. They say they will consider lending the Book, but only after they see proof that Rhysand isn’t the monster everybody believes him to be. Rhysand decides to reveal his hidden city as proof, using an orb they steal from the Court of Nightmares. Preparing for war, the group relocates to an Illyrian camp to train. One day in the forest, Lucien appears and tries to persuade Feyre to return to Tamlin. She refuses and shows him her dark side, growing Illyrian wings. That night, while staying at an inn, Feyre and Rhysand almost make love.  

The next day, Rhysand is shot down with poisoned arrows and captured by the King of Hybern’s forces. Feyre rescues him, then traps the all-knowing Suriel to ask how to heal him. The Suriel tells her, and also reveals that Rhysand is her mate and has known it for some time. Furious about being deceived, Feyre stays in a cabin, processing the news and painting the walls with images of her Night Court friends. When Rhysand finds her, she welcomes him and they complete the mating bond before he tells her that the queens have agreed to meet again. In the meeting, the queens refuse to hand over the Book, but the golden-haired queen, convinced by Rhysand and Feyre’s commitment to one another and hoping to acheive peace between the human and faerie worlds, smuggles the Book in and leaves it behind for Rhysand and Feyre to find. Soon after, Hybern forces attack Velaris, and Feyre helps the Inner Circle defend the city. With the Book decoded, Rhysand and Feyre plan to infiltrate Hybern and nullify the Cauldron.  

When the Inner Circle finds the Cauldron, Feyre is overcome by the Book’s power and cannot speak the words to destroy it. Jurian appears and distracts them while the King of Hybern casts a spell to stifle their magic. In the throne room, Tamlin and Lucien show themselves, having sold out to the king to save Feyre from Rhysand. The mortal queens are also there to claim the immortality he promised them. To prove to the queens that the process is safe, the king throws Nesta and Elain, whom he captured, into the Cauldron, turning them into High Fae against their will. Staring at Elain, Lucien announces that she is his mate. Meanwhile, Feyre secretly gets free of the King of Hybern’s spell and opens the wards for the escape of the Inner Circle, Nesta, and Elain, whom the king plans to use to gain leverage over Feyre and Tamlin. Looking at Rhysand, and understanding each other through their bond, Feyre pretends that Rhysand had put her under mind control but that she has freed herself from it. Pretending that Rhysand brainwashed her to keep her at the Night Court, Feyre asks Tamlin to take her back. She even begs the king to break her bond with Rhysand, and he does so, but the king only breaks the bargain, not their mating bond. Rhysand and the Inner Circle escape to Velaris with Nesta and Elain. As Rhysand prepares for war, Feyre returns to the Spring Court with Tamlin, who does not realize that Feyre, having been made High Lady of the Night Court only the night before, is now Rhysand’s equal in every way, as well as a willing spy.