It has been over one hundred years since the Speràdo family had an heir capable of controlling the darkness. To help his eldest daughter strengthen her weak shadow powers, Marquis Speràdo sacrifices his second-born Leslie, whose sole purpose in life was to be useful to her sister. However, mysterious beings rescue Leslie from death, and she awakens shadow magic of her own that even surpasses her sister's ability.

Fed up with her abusive family and their repulsive practices, Leslie decides to cut ties with them. To accomplish this, she seeks to strike a deal with the head of the Salvator duchy, Azela Benkahn Salvator, also known as the "Monstrous Duke." In exchange for a normal life with Azela's family until adulthood, Leslie proposes to serve as a "Dark Saint" under her wing. Determined to escape from the Speràdos' clutches, she will do whatever it takes to become a Salvator and seize happiness for herself.