The first book in the Heartstring Duet series, this dark fantasy romance is set in the kingdom of Issoria, where a mysterious pestilence called the rot kills people, animals, and crops alike. In the city of Marrowbrae, Elara works alongside her mother and younger brother Daron preparing corpses for burial. Daron, not yet 15, already shows signs of rot: a fingertip amputated, nails falling off. After a house call, a well-dressed stranger lingers outside, making cryptic remarks about "the crown" needing to be fed. Elara dismisses him.
The next morning, after the family home has been robbed and their father lies bedridden coughing blood, the stranger reappears at the graveyard. He tells Elara the kingdom's pestilence stems from a curse on the royal crown, forged by Death himself from a string torn from his own heart. The crown grants the king prosperity and immunity from death but demands a queen's blood roughly every 15 years. King Kael refuses to feed the curse, believing he can starve it into breaking, and this stubbornness is why the realm rots. At dusk, the stranger identifies himself as Vale, the king's steward, and explains his plan: Elara will enter the palace as Kael's caretaker, make him fall in love with her, and ultimately become the sacrificial queen. The sacrifice requires genuine heartbreak from the king at the moment of her death. Elara agrees, motivated by saving Daron.
The palace is rotting from the inside. Miss Hampshire, the head of staff, escorts Elara to Kael's chamber and warns that knowledge of the crown's powers is punishable by death. Elara learns Kael refuses food and sends meals to orphanages, contradicting her expectation of a selfish king. Her first encounter with Kael is brutal: His face and body are devastated by rot, his crown fused to his skull. When Elara offers water, Kael lances a chest pustule, drops maggots in the cup, and backhands it into her face. She gags and flees. Vale tells her to try again and reveals she will eventually need to bed and wed the king.
Over the following days, Elara persists, enduring Kael's hostility and talking back to him. His condition slowly improves. She takes him outside for a nighttime garden walk, and he begins to open up, recalling childhood chess games with his mother. They pass a fountain topped with a statue of Death, erected by Kael's father, King Merrick. Kael explains the crown-transfer ritual: The son slits the father's throat once the crown is lifted. He admits he brutally killed Merrick during this rite. When Elara probes further, he shuts down.
Elara investigates the palace's secrets. She discovers an abandoned royal chamber with a bloodstain hidden under a rug, which Vale identifies as belonging to Queen Maeryn, Merrick's second wife. In the library, Elara finds a half-hidden page describing the coronation sacrifice of Queen Ophelia, Merrick's third wife. In the account, Ophelia accused the king of having slaughtered Maeryn and made "his son, the prince, watch." Elara questions why Ophelia referenced a prince witnessing Maeryn's death when Kael had not yet been born, suggesting the existence of an older heir. Maeryn's annals have been removed from the archives, with dust outlines indicating recent theft. During the visit, Vale catches Elara when she falls, and they share an intense kiss he abruptly breaks off.
At a salt spring on the palace grounds, Elara and Kael share a vulnerable evening. When he moves to kiss her, Elara flinches, seized by a fear of physical intimacy, and Kael misreads her reaction as disgust at his appearance. On the walk back, he mentions his mother was allergic to roses and pears. Elara realizes the greenhouse, filled with roses and bearing a plaque dedicating it to "the queen on the birth of the prince," could not have been for an allergic Ophelia, pointing to a different queen and a different prince. Returning to the royal chamber, she discovers a child-sized handprint in the dried blood.
Elara confronts Vale, who admits to being Kael's older half-brother, the erased firstborn heir who watched his mother Maeryn die. He reveals his true plan: He wants Elara to persuade Kael to lift the crown so Vale can kill his brother, seize the crown, and become king. He promises to then marry and sacrifice Elara to feed the curse. When Elara rejects his claim of caring for her, Vale coldly tells her that her father died that morning.
Grief-stricken, Elara considers leaving, but Kael emerges into daylight for the first time to comfort her and offers to bring her family to the palace. She agrees, resolving to outmaneuver Vale while finding a way to save Daron. Their relationship deepens: Elara initiates a kiss, and Kael responds with desperate desire. A messenger arrives, and Elara eavesdrops on fragments including "heritage," "original translation," and Kael's warning that "the bastard" must not find out. She then sleeps with Vale, framing it as preparation for intimacy with Kael. Afterward, Vale tells her the origin of the curse: Death befriended an old ferryman named Eamon, lost a chess game to a king because he could not sacrifice his queen piece, and watched Eamon beheaded. In grief, Death tore a string from his own heart and wove it into the curse. Elara shares the messenger's words with Vale, who reacts with urgency and disappears.
Elara's family arrives at the palace. Daron is skeletal, coughing gray-black fluid. Desperate, Elara attempts to seduce Kael, but he refuses, insisting he will never take a queen. She glimpses a letter on his desk: "Hide her, and hide her well. Prepare the rite." Kael threatens her family if she tells anyone. Vale returns and reveals he has relocated the woman Kael was hiding, a distant relative the king intended to use in an alternative rite.
A drunk and enraged Kael confronts Elara and demolishes everything she believed. Maeryn was his own birth mother, the cold woman who never looked at him. He watched his father slit her throat as a small child. Ophelia came later as his loving adoptive mother. After Ophelia died too, Kael erased Maeryn from his lineage. Then he reveals that Vale is not his half-brother: Vale is Death himself, walking in human form. He tells Elara to lure Vale into moonlight to see his true nature.
Elara tests this during intimacy with Vale, yanking open a curtain. Moonlight exposes a monstrous transformation: pallid corpse-skin, exposed bone, a skull-like face with black eye sockets. Elara screams and hides. At dawn, Kael finds her and rushes her to the throne room, where he says he has discovered how to end the curse at its source but cannot do it himself. Death bursts through the doors, roaring that the curse cannot be broken. Instead of cutting Elara's throat, Kael presses the knife into her hand, lifts the crown from his own head, and slams it onto hers. He begs her to kill him. As Death charges, Elara slashes Kael's throat. He collapses on the dais. Elara stands over his body, the crown pulsing on her head, and faces Death with the words "Crown me dead." The story continues in the next installment of the series.