The following contains spoilers for A Thorn among Fae. Read at your own risk.
Anova is a thief and con artist living in the bustling shipping city of Irbess. Irbess is the closest human-controlled land to the fae territories adjacent to it, and the city is heavily taxed by the fae. Anova and her childhood best friend Juras are deeply in debt to the madam of the brothel their mothers used to work at. After Anova’s mother’s murder by the fae some years ago, she is tasked with the debt incurred to the brothel from their lodgings and other living expenses.
In order to raise the money to pay off their debts and leave the city on a ship for greener pastures, she and Juras operate a con most nights at different taverns throughout the city. They pretend to be engaged, stage a fight in which Anova is dumped by her cheating fiancé, and fish for men who would prey on a drunken girl. Once Anova gets the men alone, she incapacitates them and robs them.
The night after a successful job, Madam Hinterfell corners them on the street, takes all they made the night before, and makes an ultimatum: pay in full before their eighteenth birthdays in a few months, or she’ll take the case to a judge who could order the two to legally pay off their debts to Hinterfell by working for her. This threat carries even more weight because Hinterfell could try to bring evidence of their crimes to light, as well.
But during their next con in a new tavern, something goes wrong. Anova’s target knows her real name and, once they’re alone, he reveals that he’s been watching her for some time. He’s revealed to be one of the fae, a young mysterious fae lord called Leander. He wants to hire her for an assassination across the border in the fae lands. He insists her skills at pretending to be someone else are vital to the job.
Anova accepts, though she doesn’t tell him she intends not to kill anyone, but to rob him blind and return to Irbess as soon as she can flee Fae. She promises Juras she’ll come back and pay their debts with the money before it’s too late.
On the way, Anova discovers that Leander intends for them to assassinate the corrupt fae High King, only the most powerful fae who lives. Unlike the moonlight magic the other fae use, the High King or Queen is granted unrivaled power through an ancient fae artifact known as the blood crown. The crown also gives the owner the right to rule over fae, and it’s passed on through murdering the previous owner.
Because of this and his fear of a militia rising against him, the High King has grown paranoid and rules Fae with oppressive restrictions. Weapons are outlawed except those carried by his guards, fae families are allowed only one child per pairing, and similarly, families are only allowed one servant at once.
Those found violating these decrees are either punished cruelly or executed.
Leander’s family, including his twin brother, was murdered by the High King. He and his fae manservant, Nerium, have formed a plot to assassinate the king and destroy the blood crown for good. According to the lore they’ve studied, fae artifacts generally don’t affect humans like they do the fae, so they theorize that a human killing the High King will end the line of corrupt fae rulers.
However, as Leander sneaks Anova inside Fae’s borders, they are stopped by three of the king’s guards. They point out that Leander already has one servant, and Leander’s plan almost fails before it begins. They are forced to pretend that they are lovers rather than a human servant and fae master. Hellmyr, Leander’s former friend who has joined sides with the king, allows them to pass after remarking that he always knew Leander had odd tastes.
Once in Fae, Leander trains Anova in the secret training grounds inside his manor. They practice using swords, fight in hand-to-hand combat, and study poisons growing from plants inside his greenhouse and their effects.
Over time, he reveals more about his family. His twin brother, Cadmus, had a reputation for war strategy and the arts of combat while Leander was the studious, bookish brother who loved plants and lore. Their father was the king’s right hand, but the king grew suspicious and ordered his family executed except for one survivor to live and spread word of his deeds.
There is also a romantic rival vying for Leander attention, the fae lady Lycasta.
On the eve of a party that they are to attend in order to gain access inside the king’s castle, Anova decides she must steal her bounty and leave that night or not at all. Inside Leander’s chest, she finds a valuable ring and a note signed from his mother to his brother, Cadmus. Their mother urges him to live on without them.
In a change of heart, Anova leaves the ring and escapes on an enchanted butterfly steed to find another fae manor to steal from. However, when she lands, she finds the next manor empty.
He has followed her, and Anova realizes all the houses in this territory of fae are empty. Leander explains that most families here were wiped out by the fae king.
In an emotional reveal, Leander admits to Anova that he isn’t who he’s said he is.
The twin boys often switched places when they grew up, and the king had intended for the other brother to be left alive. In other words, he was supposed to have died instead of his brother.
Leander has lived the rest of his life since his family’s murders as his dead twin brother. In reality, he is Cadmus, the tactician. Leander was supposed to have been left alive as the harmless son, and he feels he stole his brother’s right to live. The High King has never found out his mistake, or so Leander (Cadmus) hopes.
This also means Lycasta has been in love with the wrong fae for years, believing him to be his dead brother, though Leander can’t correct her. He has instead rejected her advances with no explanation.
At last, Anova agrees to help him assassinate the king. At the castle, the High King reveals he will invade the human lands in less than two weeks. The two are unable to kill him though their plot remains undetected by him.
Anova, Leander, and Nerium regroup. They discover a story in a book that explains the origins of the blood crown in the form of a fairy tale.
In the fairy tale, a human witch’s son fell in love with a fae princess. Her father forbade the match, but the two met every night in the woods by the light of her lantern. The-then king discovers this, and enchants his daughter to sleep during the night while he takes the lantern to the woods. He leads her human lover over a chasm where he falls to his death.
The boy’s mother, the witch, discovers what has happened, and puts a deadly curse on the ruling fae. Because of this, the princess wakes in time to find out, and in a fit of grief and wrath, murders her own father. Left with no other heirs, the fae people are forced to recognize her as their new ruler. This is what causes the curse on the crown and the bloody succession that follows.
When the High King arrives at their manor and finds evidence incriminating only Anova, Leander orders her back to the human lands in punishment, but, unbeknownst to her, also to save her life. He doesn’t realize he’s delivered her straight to Madam Hinterfell.
With Juras’s help, she escapes prison in Irbess and Hinterfell’s clutches. Even with a broken heart, Anova realizes she’s the only one who can stop the fae invasion and end the reign of corrupt fae. She crosses the border into Fae again, and Nerium explains that Leander was trying to save her life when he banished her rather than punish her for seemingly trying to poison him.
Nerium says they discovered that their plan wouldn’t work. The end of the fairy tale explains that the witch had to sacrifice her own life to complete the curse. Ultimately, the blood crown is incompatible with humans, and would kill them if one got control of it.
Nerium also says that Leander has chosen a terrible fate to accomplish his goals—he plans to kill the High King, and then eliminate himself in hopes of destroying the blood crown with the deed. The two leave for Wolfsbane manor where the king’s army is stationed in order to infiltrate his ranks and save Leander. Nerium suggests an alternate plan—that if two fae killed the king at once, it may destroy the crown intent on only one of them becoming its master.
They talk with Leander, who is pretending to be the king’s general in the war they march to. He agrees to their plan, and they enact it during a feast the night before the invasion should start.
However, it’s a trap. The High King reveals he knows that Leander is actually Cadmus. He poisons Anova with a hallucinogenic and orders Hellmyr to kill her. Instead, he fakes her death and locks her inside a closet to save her while the army leaves.
When she wakes, Anova rides to stop the fae army and its king. Even if it means her death, she decides this is her destiny. They’ve gathered at the border between their lands. With the help of one of the king’s consorts, Letharia, Anova disguises herself as the fae lady with the intent of poisoning him.
Not yet known to the king, Letharia reveals that Hellmyr is her and the king’s son. He is the only blood heir to the crown, and she plans on supplanting the king with her son once the crown is destroyed.
The fighting between the fae and humans has started. The High King has thrown the beaten Nerium and Leander into a cage with human children and lit it on fire to persuade the humans to give up. To save Leander and both their worlds, Anova applies wolfsbane poison to her lips and kisses the king when he thinks she’s Letharia, thereby poisoning him.
The poison also affects her, however, and she collapses. In the meantime, Leander gets himself and the other prisoners out of the cage. Hellmyr runs his father through with a sword, dealing the killing blow to the king. Leander discovers that Anova’s body has gone cold. He’s too late.
But instead of answering to Hellmyr, the blood crown chooses Anova. She lives.
When she wakes, Anova realizes what’s happened. She flees a house she’s being kept inside by an unknown party, fearing for her life now that she’s the biggest target in all of Fae.