Feyre was married in secret to Rhysand and is now High Lady of the Night Court. She returns as a spy to the Spring Court with Tamlin, pretending that she had been brainwashed into staying with Rhysand in the Night Court for all those months. Secretly, Feyre is seething with anger and plots revenge.
Tamlin agreed that in exchange for Hybern’s help in getting Feyre back from Rhysand, he would open the Spring Court’s lands to Hybern’s invading forces. Jurian, the human commander who was resurrected by the magic Cauldron, along with two royals from Hybern, arrive in the Spring Court to inspect the magical wall separating the human and Fey lands.
In Spring, Feyre is revered as the Cursebreaker, the one who ended Amarantha’s vicious rule over Prythian. Feyre sows distrust throughout Tamlin’s court and undermines the high priestess, Ianthe. Feyre argues with Tamlin, purposely inciting his temper, and when he causes the room to explode, Feyre doesn’t shield her body. Tamlin’s court sees Feyre’s cuts and bruises, caused by flying debris. They begin to hate Tamlin for his abuse. She also incites Tamlin’s jealously by confiding in Lucien, Tamlin’s best friend.
Feyre accompanies Jurian and the Hybern royals to the wall to check for holes in the magical barrier. The royals reveal that they are looking for places they can use the Cauldron to destroy the wall, allowing Hybern to capture human lands and enslave the people. With this information, Feyre is ready to leave Spring Court and return to the Night Court.
In the woods, she finds Ianthe sexually assaulting Lucien. Instead of leaving as she had planned, Feyre stops to help Lucien. Feyre breaks into Ianthe’s mind and compels the priestess to stop, but the Hybern royals find her and reveal that they have been poisoning Feyre for weeks. Feyre and Lucien fight and kill the royals, but her physical and magical strength is sapped from the poison. She agrees to take Lucien with her to the Night Court.
They travel through the Autumn and Winter lands, where Lucien’s brothers find and try to kill them, but Azriel and Cassian of the Night Court rescue them. Back in the Night Court, Feyre is reunited with Rhysand and overjoyed to see her friends have healed from the confrontation they had with the king of Hybern. Her sisters, who were forced into the Cauldron and transformed into High Fae, are angry, depressed and cold to Feyre.
Elain speaks strangely of things that haven’t happened yet, and Nesta possess an otherworldly presence that none can understand. The Cauldron has made Elain a seer, while Nesta went into the Cauldron and stole death from it. Since Hybern intends to put the Cauldron in a weak spot in the wall to destroy it, Feyre asks Nesta to use her connection to the Cauldron to patch the holes in the wall. Nesta and Amren search for useful spells in a book of magic.
Since Prythian’s armies are much smaller that Hybern’s, Feyre asks monsters for help. The Bone Carver is an ancient being of death that wandered into the world, along with his sister the Weaver. Feyre asks him to fight alongside them in the war, and he agrees, but only if she can fetch the magical Ouroboros Mirror for him. The mirror forces a person to see your true self. Those who have tried to take it have been driven mad.
The Carver will need a Fae body to leave his prison. Another creature, Bryaxis, agrees to leave its prison in a pit to help fight the upcoming war. Elain sees a vision of a mortal queen who is cursed to turn into a firebird every night. Lucien volunteers to go find the queen and ask for her help in the upcoming war.
Rhysand calls a meeting of all the high lords, but before they convene, Hybern attacks the Summer Court. The Night Court goes to help and finds a slaughter in progress. Feyre and Morrigan fight through all the soldiers they encounter, while Rhysand tries to reach the king, who uses magic and illusion to evade capture. Hybern withdraws and the fighting ends, but the casualty count is high. Hybern’s forces are considerably larger than Prythian’s, and all the Fae courts must be united for them to stand a chance against so large an enemy.
The meeting of the high lords is held in the Dawn Court, a neutral territory. All the lords participate, including Beron from the Autumn Court and Tamlin. Both verbally attack Feyre and Rhysand, accusing them of working with Hybern. Tamlin is particularly bitter about Feyre’s betrayal. Her plotting left his court in shambles. As a result, the Spring Court has been taken over by Hybern, and its people were forced to flee to other courts. Nesta’s connection to the Cauldron makes her ill with a sense of foreboding, just before a huge wave of power sweeps across the land and the Cauldron brings down the wall.
The high lords, except for Beron and Tamlin, pledge to work together against Hybern, and they all leave for their respective courts. Feyre and her family travel to the human lands to warn them of the coming threat. Elain glamours herself to look human so she can plead with her fiancé to take as many people as possible into his father’s fortress.
When Graysen sees Elain, he knows she is Fae and rejects her. Jurian is also there to warn the humans about the war. He is not the enemy, but instead has been spying on Hybern, working to undermine the king and save the humans. He tells Rhys that Tamlin went back to Hybern and betrayed the high lords, and Hybern is planning to attack the Summer Court again.
Using Jurian’s information, the Fae ambush Hybern. There is a bloody battle, but these soldiers don’t make up the bulk of Hybern’s army. Feyre goes to ask a magical creature, the Suriel, if it can see where the king of Hybern and the rest of his army are hiding. The Suriel tells her that the Cauldron cloaks the king’s location, but Nesta, with her connection to the Cauldron, can find both.
The Suriel also tells Feyre that Amren and a spell in the book of magic are the key to winning the war. Ianthe finds Feyre and kills the Suriel. She pursues Feyre through the woods, and Feyre traps the priestess and her soldiers inside the Weaver’s cottage, where the monster eats them. Helion finds Feyre in the forest and takes her back to the war camp. Nesta opens a link between herself and the Cauldron and finds its location, but the link goes both ways and the Cauldron sees their location, too. In the night, Elain is kidnapped and taken to Hybern’s camp.
Feyre dons Ianthe’s appearance to sneak into Hybern’s camp to rescue her sister. She receives help from Jurian and Azriel, and as she flees with Elain, the king sends his hounds to attack. Tamlin, who has been secretly working against Hybern from the inside, shapeshifts into a lion and saves their lives, allowing them to escape.
Feyre retrieves the Ouroborus Mirror, and the Bone Carver pledges his help. As both sides prepare for battle, the Bone Carver, the Weaver and Bryaxis join the front lines. Surprisingly, Beron, Tamlin and Graysen’s armies join the battle against Hybern.
Feyre’s father and Lucien found the firebird queen, and her army joins the fight. The armies of Miryam and Drakon, Fae who fought in the last war and have been in hiding ever since, come to Prythian’s aid as well. Even with the additional help, the fight does not go well for the human-Fae army. The Cauldron destroys the Bone Carver and many of the Night Court’s army.
Nesta amasses her power as a distraction for the king, while Amren and Feyre go to the Cauldron to nullify its power, which Amren said they could do. Feyre touches the Cauldron, and its magic traps her. Amren goes into the object. Amren lied to Feyre. The spell isn’t to nullify the power in the object. Amren uses it to unbind herself from the body she is trapped in.
Nesta and Cassian face off against Hybern. The king has captured Nesta’s father and kills him. As the king is about to kill Nesta and Cassian, Elain comes from behind and stabs him in the neck. Nesta ends the king’s life by beheading him.
Despite the king’s death, the battle rages on. Amren transforms into her true self, a winged, haloed creature of blinding light. She unleashes fire and brimstone on her enemies before burning out. The Cauldron is broken and starts to pull everything into its nothingness.
Feyre tries to fix it before it destroys their world. She uses a spell, but her power is not strong enough. Rhysand gives her his power, and in doing so, knowingly gives his life. The Cauldron is fixed, but Feyre is distraught and demands that the other high lords resurrect him, like they did for her. They all do, including Tamlin, and Rhysand is brought back to life. When Rhysand died, he saw Amren in the Cauldron and asks if she wants to come back. She did and is given a resurrected High Fae body.
After the battle, Feyre, Nesta and Elain burn their father’s body and pray for him to enter the afterlife. Feyre asks Miryam and Drakon to hide the Cauldron on their island. Vassa, the firebird queen, asks Feyre to work on a spell that can break her curse. Feyre calls a meeting to discuss a new treaty between humans and Fae before she, her friends and family return to Velaris and the Night Court.