The Angel and the Devil Both Want My Heart follows Kassiane Ferro, a 21-year-old bellmaker from the coastal city of Salteras who possesses a strange ability: she can hear the hidden frequencies beneath sounds and detect when something—or someone—is lying.

On her twenty-first birthday, an ancient bell rings with two voices, announcing the arrival of two supernatural claimants. Barachiel, an angel and executor of Heaven, presents a contract signed by Kassiane's deceased mother, while Dantalion, a powerful duke of Hell, reveals a second contract signed by her missing father. Both contracts claim Kassiane's heart, and she is given forty-nine days to freely choose between Heaven, Hell, or neither.

As the two supernatural beings court her under strict magical laws, Kassiane discovers that the contracts may conceal something far more dangerous. When a mysterious Auditor named Zaphkiel arrives to question whether she is legally a person rather than property, Kassiane realizes that her real battle may not be choosing between an angel and a devil, but proving that her heart—and her life—belong to her.