On the outskirts of a small town, Leah lives quietly with her mother, her only family. That peace is shattered when a beast controlled by an unknown entity kills her mother. Leah is left utterly alone, helpless in a blood-soaked room, at the mercy of a mysterious man dressed entirely in black. This figure, a reaper, offers her a chance: “Do you swear on revenge?” Leah agrees, forging a pact with the inhuman being who looks like a boy. Their bond, a mix of love and vengeance, drives the story forward as Leah vows to hunt down those responsible for her loss. The manga follows her journey through a dark fantasy landscape, where her quest for retribution pits her against the forces that control the beasts, while her relationship with the reaper deepens in unexpected ways. The narrative balances action and emotional turmoil, exploring how far Leah will go to fulfill her vow and what she is willing to sacrifice in the process.