Orphaned as a child and raised in an earls family, Wisteria Irene grew up alongside Bright, the son of a ducal house. She secretly loved him and devoted herself to studying the Predawn Realm, a dangerous underworld filled with miasma that is also the source of all magic, hoping her knowledge might prove useful to him since he could not use magic. Her stepsister Rosalie, however, was the one who captured Brights heart. When Rosalie is chosen to become the guardian of the Predawn Realm, a role that means certain death, a distraught Bright begs Wisteria to take her sisters place. Recognizing that her love for him will never be returned, Wisteria agrees to the sacrifice.

She enters the Predawn Realm expecting to die alone, but she survives with the help of the sacred sentient sword Sartis, who takes pity on her circumstances. For 23 years, she lives in isolation within this frozen world, fighting monsters as its guardian. Although she is now in her forties, her appearance has not changed since she entered the realm at age twenty. In the world she left behind, she becomes feared as a witch.

Her solitude ends with the arrival of Lloyd Allen-Wruining, a 22 year old young man who bears a striking resemblance to Bright. He is the son of Bright and Rosalie, making him Wisterias nephew by marriage. Lloyd declares his intention to defeat Wisteria and claim the sacred sword Sartis, but he also proposes becoming her disciple. The story follows the complex relationship between the wounded and lonely former noblewoman and the young man who seeks to master the sword she wields, as Wisterias long frozen destiny begins to move once more.