Celaena Sardothien is known as the deadliest assassin in the kingdom of Adarlan, having spent her life being tutored by the King of Assassins, Arobynn Hamel, and countless other masters across the continent of Erilea. However, for the past year Celaena has been enslaved in the Salt Mines of Endovier, a prison labor camp where the survival expectancy for the average inmate is three months. The book opens on Celaena being taken out of her work schedule by Chaol Westfall, Captain of the Royal Guard, and brought to meet with the Crown Prince of Adarlan, Dorian Havilliard. Dorian tells her he wishes to sponsor her in a competition held by his father, the King of Adarlan. If she wins the competition, she will serve as the King’s Champion (that is, his personal assassin) for four years before being granted her freedom. She must compete in tests against twenty-three other tributes and win a final duel, and if she fails, she will be sent back to Endovier to live out her sentence and die. Enraptured at the thought of being free, Celaena agrees. 

Celaena, Dorian, Chaol, and their company go to Rifthold, Adarlan’s capital and the home of the evil King of Adarlan. Ten years ago the king ordered the ridding of all magic, slaughtering countless Fae people and corruptly imprisoning any practitioners of magic; he is a tyrannical and ruthless ruler, and Celaena both fears and despises him. The royal castle in Rifthold is made out of glass, but Celaena is shown to her chambers in the stone part of the castle, where she meets Philippa, her head servant, and Kaltain Rompier, an unlikeable court lady whose eyes are set on becoming Dorian’s bride. Celaena feels on edge at the prospect of being surrounded by the royalty that she despises, but basks in the comforts that were denied to her in Endovier. 

Under the alias Lillian Gordaina, Celaena begins training alongside the twenty-three other Champions, out of which she is the only woman. She struggles to gain her strength back after her enslavement, but works extraordinarily hard under the mentorship of Chaol. They slowly grow to like and trust each other, but constantly bicker over what Celaena is allowed to do in the castle. Dorian and Celaena also become friendly and flirtatious with one another, and Celaena begins to have feelings for him. However, danger grows when one of the Champions, Bill Chastain, is found gruesomely murdered. 

Celaena meets Nehemia, the Princess of Eyllwe, who says she is staying at Rifthold to learn the common tongue and customs of the city. Celaena learned to speak Eyllwe from imprisoned rebels in Endovier, who helped her get through the torturous first months of her enslavement; she tells Nehemia only that she learned the language from an Eyllwe woman, and the two strike up a friendship. Eyllwe is the last country to resist being seized by Adarlan, and Celaena supports their cause, as she hates the king and his corrupt rule. Celaena also gets to know the other Champions—she despises the monstrous Cain and his lackeys, Grave and Verin, and comes to like Nox Owen and Pelor, the other seeming underdogs. 

Celaena passes an archery test with ease, toning down her skill so as to fake meekness and not become a target. She also passes the climbing test, and saves Nox’s life at great risk to her own. She continues to pass the tests easily. Meanwhile, Dorian’s mother, Queen Georgina, frustrates Dorian by pestering him to look for a potential bride, and Kaltain gets closer to the vicious and power-hungry Duke Perrington, hoping to manipulate him into helping her get rid of Celaena, who she believes is after the prince.

On the holiday Samhuinn, Celaena and Nehemia find Cain drawing what Nehemia identifies as  “Wyrdmarks” in the dirt. She tells Celaena not to look into them, and suggests that Celaena teach her the common tongue in exchange for lessons in Eyllwe. That night, Celaena discovers a hidden passageway in her room that leads down into the castle’s depths. There she finds an unguarded escape route out of the castle, but doesn’t use it yet. She also finds the tomb of Queen Elena and King Gavin, the first rulers of Adarlan. Elena comes to her in a dream later on, to tell Celaena that there is a great evil in the castle, and that Celaena must become the King’s Champion. 

Two more Champions die from brutal maulings, and each crime scene is etched with both Wyrdmarks and claw marks. Celaena helps Chaol investigate, and that night she dreams of Elena telling her again to win the competition and to “keep an eye on her right.” Celaena begins obsessively studying Wyrdmarks in hopes that she will figure out what is going on. Dorian begins visiting her rooms at night when she is not researching, and the two grow closer. 

Three more Champions are murdered over the course of the following weeks, and the final duel nears. Celaena runs into Nehemia in the library one night, and after seeing a sketch of a Wyrdmark that Nehemia left, begins to suspect that she is the murderer. Worried that Nehemia will attack the court at the Yulemas ball, Celaena sneaks into the party to keep watch on her. Nothing happens, but that night Dorian comes to Celaena’s rooms and kisses Celaena for the first time.

A few days before the final test, Duke Perrington reveals to Kaltain who Celaena really is, and plots with her to poison Celaena’s wine during the final duel to weaken her against Cain. That night, Celaena discovers vital information on the Wyrdmarks: they can be used to summon a monster called a ridderak, which will murder the summoner’s victims and grant the summoner the victim’s strength. Celaena goes down into the hidden passageway and finds Cain, not Nehemia, summoning the beast. Cain locks her in the passageway with the ridderak, and Celaena kills it using the sword in Elena’s tomb. It bites her before it dies, and Celaena stumbles back to her room, where Nehemia finds her and heals the deadly wound. When she regains consciousness, Celaena tells Nehemia who she really is and why she’s at the castle, and Nehemia reveals that she has known about Wyrdmarks the whole time. 

The next day, Nox figures out Celaena’s true identity, and she tells him to escape the castle that night so as not to be killed by Cain. He does, causing Brullo, the judge, to cancel the last test and schedule the final duel for two days hence. That night, Dorian visits Celaena’s rooms and they kiss and talk long into the night. 

The day of the duel, Cain beats Renault and Celaena beats Grave before Celaena and Cain face off. Nehemia gives Celaena her staff to fight with, and the Champions drink ceremonial wine before the final duel. Cain and Celaena begin to fight, but Celaena quickly realizes that she has been drugged, becoming more and more incapacitated. As Cain beats her up, Celaena hallucinates that she is in a shadow world, and Cain is leading an army of the dead and demons to attack her. Just as he is about to finish her off, Elena appears and defends her, holding off Cain and the army and removing the poison from Celaena’s system. When the visions end, Celaena stands and charges Cain, stabbing him and winning the duel. After being pronounced the winner, she collapses into Dorian’s arms. Cain attempts to kill her while her back is turned, but Chaol kills him before Cain can strike.

While recovering, Celaena is visited by Nehemia, who tells her she saved Celaena’s life at the duel by opening a portal for Elena to come through, and that she’s been using Wyrdmarks and Wyrdgates this whole time to protect Celaena from the ridderak. Later, Dorian visits to tell Celaena that he wants to be with her, and Celaena refuses, saying she wants to be totally uninhibited by the time she is granted her freedom in four years. She signs the king’s contract, and walks back to her room arm-in-arm with Chaol.