Set in an alternate Joseon dynasty, the story follows Kim Sung-yeol, a noble scholar who is turned into a vampire after a tragic incident involving his family and his betrothed. Haunted by his past and the loss of his humanity, he spends the next 120 years living as a 'guardian' vampire, hunting down other vampires who threaten the human world. He specifically seeks to destroy Gwi, a powerful and malevolent vampire who resides in the palace and manipulates the royal family from the shadows. His mission takes a turn when he encounters Jo Yang-sun, a young woman who disguises herself as a male bookseller to support her family. As Sung-yeol becomes entangled with Yang-sun and the exiled Crown Prince Lee Yoon, he discovers that Yang-sun holds the key to a secret plan that could finally defeat Gwi. Together, they must navigate political conspiracies, forbidden romance, and the struggle between their human desires and supernatural fates.