Months have passed since Feyre saved the Fae kingdom of Prythian from the vicious queen Amarantha. Feyre now lives with the magic gifts the High Lords of all seven Fae courts gave her. Immortality is one of them. She is still bonded to the Rhysand, High Lord of the Night Court and obligated to spend one week of every month with him. So far, he has not come to collect on the debt.
Feyre is now engaged to Tamlin and lives with him in his estate in the Spring Court. Instead of being happy, Feyre suffers from depression and harrowing nightmares of the innocent faeries she killed during the trials. Her relationship with Tamlin is strained as he has his own nightmares but refuses to open up to Feyre, choosing instead to keep her secluded in the manor under heavy guard.
Feyre wants her freedom and to learn to fight, but Tamlin refuses and tasks her with preparing for their wedding and becoming a proper hostess for visitors to the Spring Court. On their wedding day, Feyre feels overwhelmed and wants to escape, knowing that she isn’t ready for marriage. During the ceremony, she has a panic attack. Rhys hears her through their bond, appears and takes her away to the Night Court for a week.
Rhys gives Feyre space and freedom. He introduces her to his cousin, Morrigan, a high Fae woman, and the two become friends. During her time there, he teaches her to read and tells her that when the High Lords gave her a bit of their magic to save her life, she also received their power and abilities. He tells her she is stronger than she knows and should be taught to use her abilities.
When Feyre goes back to Tamlin’s manor, she is overjoyed to see him. Instead of wanting to be intimate with her, however, Tamlin grills her on everything she saw and heard in Tamlin’s court. Feyre feels like a spy, but since Tamlin and Rhysand are enemies, she does what Tamlin wants.
Feyre asks him about her new abilities. Tamlin admits that he knew she had them but they are too dangerous for her to train. He only wants to keep her safe and protected from those who will want her power for themselves.
As the weeks pass, Tamlin becomes more controlling, and Feyre becomes even more despondent. One day as Tamlin is leaving to assess a new threat in his lands, Feyre insists on going with him. Tamlin casts a shield around the manor, imprisoning Feyre inside. She has flashbacks of being trapped Under the Mountain by Amarantha and passes out. Rhysand rescues her and takes her to his secret city of Velaris.
Rhysand keeps two courts: the public Hewn City, a place of wickedness and debauchery, and Velaris, a secret, peaceful seaside city of beauty, art and culture. Velaris has been hidden with spells for hundreds of years, and that is where Rhysand and his friends call home. Feyre stays at Rhys’ townhome in Velaris where she slowly recovers from her depression and heartbreak, vowing never to go back to Tamlin and be imprisoned again. She sends Tamlin a letter telling him that she wanted to leave, she’s fine, and she won’t be coming back.
Rhys is suspicions that the king of the neighboring Fae kingdom of Hybern is preparing to attack Prythian. The king has been unhappy with the treaty to end human enslavement and wishes to break down the wall between Prythian and the mortal world. Rhys asks Feyre to work with his inner circle — Morrigan, Amren, Azriel and Cassian — to find out what the king is planning. While Azriel and Cassian teach Feyre how fight, Rhys teaches her how to use her new powers.
To get more information, Feyre and Rhys visit an ancient creature called the Bone Carver. The creature tells them that the king is reassembling the Cauldron, a magical object of immense power from which their world was created. It was stolen and hidden, but the king now has it and plans to use it to raise a dead warrior, Jurian, to shatter the wall to the human realm. The only thing that can stop the Cauldron is the Book of Breathings, half of which is in Prythian at the Summer Court and the other half is in the human realm with the six human queens.
Rhysand and Feyre travel to the Summer Court under the guise of diplomacy and steal that portion of the book; but the half in the human realm can’t be stolen, it must be given freely. They arrange a meeting with the human queens at Feyre’s family home in the human realm. Negotiations with the queens go poorly. They refuse to turn over their half of the book as they have heard about Rhysand’s wickedness and don’t trust him. They require proof that he will use the book for good and not evil.
Feyre’s group returns to Prythian and steal a device, the Veritas, which allows the viewer to see only the truth. Rhys plans to use the Veritas to show the queens Velaris so they can see that he is good, capable of maintaining peace and worthy of their trust. His public image is a role he plays and not who he really is.
In the months since she left Tamlin, Feyre finds that she is getting over her heartbreak and falling in love with Rhys. While they wait for the queens to meet with them again, Feyre continues her training with Rhys. While training in the woods, they are ambushed by Lucien, Tamlin’s emissary, who tries to forcibly take her back to the Spring Court. Feyre uses her considerable power and makes it clear that she will never return.
A few weeks later, the king of Hybern attacks them, and Rhys is badly wounded and taken captive. Feyre finds him, but she does not know how to heal him. She leaves Rhys in a cave and goes into the woods to set a trap for a Suriel, a faerie who is compelled to tell the truth when captured. The Suriel tells Feyre that Rhys is her mate, and to give him her blood to drink as it has healing properties. Feyre is shocked, but the Suriel confirms that Rhys has known Feyre is his mate, his perfect match and one who he is destined to be bonded with for life. Feyre is furious that Rhys held the truth from her, and after she heals him and takes him to safety, she leaves for a remote cabin.
Eventually, Rhys finds her and tells her that he felt she was his mate when Amarantha killed her, but it was confirmed when he saw her in her Fae form for the first time. Feyre forgives him and accepts the mating bond. They have sex, and their bond is sealed.
When they go back to the human realm with the Veritas, they hope the queens will give them the book as time is running out to stop the king. Morrigan shows them the secret and thriving city of Velaris. While the queens are astonished, they still refuse to give their half of the book. Feyre and Morrigan are angry, but when the women leave, Rhys sees that one of the queens secretly left the book for them with a note telling them to not trust the other queens.
The group returns to Velaris. Flying creatures from Hybern attack the city. Feyre and the others defend the city, but not before considerable damage is done and lives are lost, including the life of the queen who gave them the book. One of the soldiers drops her from the sky, and she dies at impact.
They decide to destroy the Cauldron. Amren deciphers a spell to break the object, warning Feyre that she must not put the two halves of the book together; the joining will create power that will attract unwanted attention. Feyre and others sneak into the Hybern castle, and they find the Cauldron.
Before Feyre can utter the spell, the book compels her to join it. The magic created alerts Jurian, the king’s newly resurrected warrior. Rhys takes the book from Feyre before the king spells the castle so no one can leave. The king joins Jurian, the human queens who betrayed Rhys, Tamlin and Lucien.
Tamlin has made a bargain with the king. If he helps to get Feyre back, he will grant Hybern safe passage into Prythian through the Spring Court. Feyre refuses to go with Tamlin, but the king has caught her sisters and uses them to show the human queens that it’s safe to be remade by the Cauldron. As Feyre begs for their release, her sisters are submerged into the Cauldron and come out as high Fae.
Lucien looks at the newly made Fae Elain and realizes that she is his mate. Feyre knows she has no choice, as she must save her sisters and her friends. She pretends that Rhys has always had her under his control.
She asks Tamlin to take her home to the Spring Court and begs the king to break the mating bond. The king obliges, unaware that he only breaks the bond to spend one week a month in the Night Court. The mating bond cannot be severed by magic. Feyre telepathically asks Rhys to teleport himself and her sisters out of Hybern. Lucien is furious to lose Elain, but he, Tamlin and Feyre return to the Spring Court. Tamlin apologizes for his past behavior, telling Feyre that things will be different between them.
Rhys returns to Velaris and tells the others that the night before they went to Hybern, he and Feyre went to a priestess, and she was sworn in as High Lady of the night court. Feyre is not his wife or consort, but his equal in every way. She is now their spy. Tamlin has unknowingly led the High Lady of the Night Court into the heart of his territory.