After returning to the Spring Court at the conclusion of A Court of Mist and Fury, Feyre lies to her former friends and allies and pretends that she was kidnapped and tortured by Rhysand, High Lord of the Night Court. In fact, Rhysand is her mate and she is now the High Lady of the Night Court, a fact which she carefully conceals from Tamlin, her former lover and High Lord of the Spring Court, as well as Lucien, Tamlin’s close friend and right-hand man. She carefully hides her anger in order to carry out her plans for revenge against Tamlin and his allies.  

Previously, Feyre canceled her engagement to Tamlin due to his controlling and abusive behavior and fled to the Night Court, where she realized that Rhysand was her true love and the two developed a mating bond. Refusing to believe that Feyre left him willingly, Tamlin decided to go to war with the Night Court to restore Feyre to the Spring Court, forming an alliance with the cruel King of Hybern, who wishes to take down the wall between the faerie and human lands in order to re-enslave humanity. Due to the machinations of the High Priestess Ianthe, Feyre’s human sisters, Elain and Nesta, were dragged into the conflict. After being thrown against their will into the Cauldron, a highly powerful magical object, Elain and Nesta were transformed into immortal faeries. With her sisters safe under the care of Rhysand, Feyre manipulates those in the Spring Court in order to weaken Tamlin’s power and undermine the alliance with Hybern.  

The King of Hybern sends his emissaries, two twin commanders named Brannagh and Dagdan, along with Jurian, a legendary human warrior who was resurrected through the Cauldron’s power, to the Spring Court to formalize their alliance and examine the wall that divides the humans from the faeries. Feyre has further enemies at court, such as Ianthe, who seeks to secure her own power through various schemes. Feyre is successful in her plans to manipulate Ianthe and others at court, and slowly she drives a wedge between Tamlin and his men, who begin to feel that Tamlin has sold out Prythian to Hybern and sided with Ianthe against his own soldiers.  

When the Hybern commanders request to examine the holes in the wall, through which they intend to take down the wall and begin their invasion of the human lands, Feyre offers to accompany them alongside Lucien. On the third of these trips, they are accompanied by Tamlin and Ianthe. After gaining useful information from the twin commanders, Feyre intends to flee to Rhysand in the north. However, she stops to punish Ianthe when she catches her sexually assaulting Lucien. Brannagh and Dagdan suddenly appear and reveal that they have been poisoning Feyre and Lucien with a powder called faebane to block their magic powers. In a fierce fight, Feyre and Lucien defeat and kill the twins.  

Lucien, who is mate-bonded with Elain, decides to accompany Feyre on her journey to the Night Court. Without their magic, the journey is difficult and dangerous. They move through the Autumn Court, ruled by Lucien’s family, which cruelly betrayed him in the past. While seeking shelter in a cave, they are found by Lucien’s brothers, who intend to capture Lucien and Feyre on the orders of Beron, Lucien’s father and High Lord of the Autumn Court. Lucien and Feyre escape and cross over to the Winter Court, where Lucien’s brothers again catch up with them. Feyre and Lucien are rescued by Azriel and Cassian, members of Rhysand’s trusted Inner Circle. Together, they fly to city of Velaris in the Night Court, where Feyre is reunited with the other members of the Inner Circle, her good friends the Morrigan, or “Mor,” and Amren. She also visits her sisters, Nesta and Elain, and finds that Elain has been greatly weakened by her experiences in the Cauldron. Feyre feels that she is not yet ready to meet with Lucien, who is allowed to stay in Velaris but is kept under close watch.  

After exchanging the information they have gathered about Hybern’s plans, Rhysand decides to call a meeting with the High Lords of the other courts to propose an alliance against Hybern. Willing to make unorthodox allies, Feyre meets with the Bone Carver, an ancient and powerful being held in The Prison, who reveals that he has accepted imprisonment willingly to hide from his more powerful older siblings, one of whom is the Weaver. Uninterested in being freed, the Carver instead requests a powerful mirror called Ouroboros, held in the Court of Nightmares. Later, they meet with Keir, estranged father of Mor and steward of the Court of Nightmares, a sub-court of the Night Court, in order to request the assistance of his Darkbringer legions and obtain the mirror. At the meeting, Keir accepts Rhysand’s request after Rhysand promises an alliance with Eris, whom they will support as the future High Lord of the Autumn Court, as well as limited access to Velaris for Keir’s people. Feyre requests the mirror, and Keir offers it to her but notes that it can only be taken by someone who is willing to look into it, an act which has driven several others mad in the past.  

Mor, who was cruelly abandoned by her father after the cancellation of her engagement to Eris, is deeply hurt by this proposed alliance, but she also understands its tactical necessity. Amren, an ancient being from another world who bound herself to the form of a faerie to escape from The Prison, reluctantly informs Feyre that she can only free the Carver if it agrees to be similarly bound to another, weaker form. The group prepares for the war. Feyre, who can shape-shift, develops wings and slowly learns to fly with lessons from Azriel. Cassian trains Feyre in combat and Amren trains Nesta, who holds a mysterious power that the group feels could possibly fix the holes in the wall.  

While conducting research in the vast library under the House of Winds, one of Rhysand’s palaces, Feyre and Nesta are attacked by two agents of Hybern, who reveal that, when Nesta was thrown into the Cauldron, she also took some of its power, weakening it. Now, the King of Hybern wants to abduct Nesta to return the Cauldron’s power.  Under pressure, Feyre makes a deal with the monster at the pit of the library, whom she learns is named Bryaxis. Bryaxis attacks the Hybern agents and Rhysand, alerted by the priestess Clotho, arrives and kills the agents. Later, they realize that Elain, who has been making cryptic and seemingly meaningless comments, is a seer who accurately predicted the attack by Hybern. Following one of her prophecies, Lucien travels to the continent to find a human queen, “cursed” by the others, who might prove a crucial ally in the war.  

Before the planned meeting, the war begins abruptly as Hybern attacks Adriata, capital city of the Summer Court, ruled by High Lord Tarquin, who resents Rhysand and Feyre for tricking and robbing him in the past. Despite these tensions, Rhysand and his Inner Circle lead an army to Adriata, where they help to repel the Hybern forces. Later, at the meeting, the High Lords reach an uneasy alliance despite the longstanding tensions between them. Tamlin, who shows up unexpectedly, insults Feyre and Rhysand but nevertheless reveals that he has been working as a double agent, collecting intel on his presumed ally, the King of Hybern. At the end of the meeting, the group is shocked when the King of Hybern uses the Cauldron to destroy the wall, beginning his attack on humanity. Feyre forges a deal with Bryaxis, securing its assistance in the war.  

Elain develops a plan to evacuate as many humans as possible to the fortified estate of her former betrothed, Graysen, a human whose father, Lord Nolan, bitterly hates faeries. The faeries travel to the estate, disguising Elain’s faerie features. They make their proposal to Nolan, who reveals that he knows about Elain’s transformation. Suddenly, Jurian appears. He states that he remains loyal to the human race and has been working against Hybern. Though Graysen cruelly rejects Elain, the faeries nevertheless secure Nolan’s agreement to shelter evacuating humans.  

Using intel from Jurian, the allied lords counter Hybern’s second assault on the Summer Court. They then fight again as another Hybern legion moves north toward the Winter Court. During that battle, Feyre decides to find the Suriel, a strange creature, who informs her that Nesta can locate the Cauldron using her connection to it and, additionally, claims that the secret to defeating Hybern can be found in the Book of Breathings, a powerful spell book in Amren’s possession. Ianthe appears and kills the Suriel and chases Feyre, who leads Ianthe to the cottage of the Weaver, who kills and consumes the priestess.  

Feyre returns to Rhysand’s camp. They have narrowly defeated Hybern’s legion, but Cassian is badly injured. Nesta uses her connection to the Cauldron to locate it and discovers that Hybern has kept the bulk of his tremendous army hidden near the human border. However, the Cauldron also senses Nesta, and it later lures Elain, taking her to Hybern’s camp. Desperate, Feyre disguises herself as Ianthe and travels with Azriel to Hybern’s camp, where they save Elain and escape with the help of Tamlin, who then flies off, having exposed his true loyalties to Hybern.  

At Feyre’s demand, the High Lords winnow, or magically transport, the humans out of the path of Hybern’s destruction as its troops move west across human lands. The allied Prythian army faces Hybern on the battlefield, strengthened by Bryaxis, the Carver, and the Weaver, who makes a bargain with Rhysand for her freedom. Though the Prythian troops are further reinforced by the armies of Tamlin, Beron, and a human army led by Jurian, Hybern uses the Cauldron as a weapon, instantly killing thousands of troops and destroying the Carver. When the tide of battle seems, again, to turn against Prythian, Prince Drakon and Miryam arrive with more reinforcements, accompanied by Queen Vassa and Feyre’s father.  

Amren, who learned from the Book of Breathings that those who have a connection to the Cauldron can withstand its power if they touch it at the same time, leads Feyre to the Cauldron while Nesta, who holds the King of Hybern’s attention due to the powers she has taken from the Cauldron, serves as bait, intending to draw him out. Feyre and Amren pass the King of Hybern as he kills the Weaver and winnows toward Nesta. Feyre puts her hands on the Cauldron and learns that Amren has deceived her. Connected to the Cauldron, Feyre sees everything happening on the battlefield. Her sister lures out the King, hoping to use her store of magic to kill him, but he uses their father as a human-shield and then snaps his neck. As the King prepares to kill Nesta and Cassian, he is stabbed through the neck by Elain, and Nesta uses the knife to decapitate him.  

Feyre returns to her own body and accuses Amren of deception. Amren acknowledges that her spell is not designed to destroy the Cauldron, but rather, to unbind herself from her faerie form, unleashing her full power. Hesitantly, Feyre recites the spell that unbinds Amren, who jumps into the Cauldron and emerges a powerful glowing entity that destroys all Hybern’s remaining troops before fading away. This act, however, creates cracks in the Cauldron that threaten to destroy their entire world. Feyre serves as a conduit, channeling Rhysand’s immense magical power into the Cauldron and sealing its cracks. However, the effort kills Rhysand. A desperate Feyre implores the High Lords for help and they all, including Tamlin, drop a bead of light onto Rhysand, bringing him back to life. Amren is also revived, re-appearing in the Cauldron in a fully faerie state, without her previous powers.  

Feyre and her sisters bury their father. Feyre meets Miryam and Drakon and then holds a meeting with the various lords in the half-ruined house once owned by her family, proposing to negotiate the treaty between humans and faeries. She also meets Vassa, who requests help breaking the curse that binds her to the death-god. Feyre and her friends and family return to Velaris, where they celebrate their success and look upon the future with optimism.