Wisteria, the adopted daughter of an earl, has harbored a secret love for her childhood friend Bright, the son of a duke. Unable to use magic, Bright secretly conducted forbidden research, and Wisteria devoted herself to studying the underworld to be of help to him. Her world shatters when her younger stepsister Rosalie is chosen as the guardian of the Predawn Realm, a miasma-filled underworld that functions as a one-way sacrifice from which no one returns.

In her despair, Bright begs Wisteria to take Rosalies place, confessing his love for her sister in the process. Realizing her feelings are unrequited, Wisteria agrees to the sacrifice. Using her natural talent for magic and her unique resistance to the deadly miasma that threatens the mortal world, she enters the Predawn Realm accompanied only by the holy sentient sword Sartis. There, she survives in solitude for twenty-three years, fighting monsters and aging no further than her twenty-year-old self.

Her isolated existence is upended when a young man named Lloyd appears, bearing an uncanny resemblance to Bright. He demands Wisterias sacred sword and challenges her, yet upon defeat, he stubbornly insists on becoming her disciple. As Wisteria finds herself shaken by even his smallest gestures, she discovers that Lloyd is not the man she once loved but her own nephew the son of Bright and Rosalie. His quest for the holy sword and his true reasons for seeking her out in the poisonous realm set the stage for a reunion that forces Wisteria to confront her past betrayal and the feelings she has buried for over two decades.