A half-beast mercenary who wants to be human guards a witch who doesn't understand humanity. Zero asks the dumbest questions because she's literally never seen the world before, and his answers, delivered with exhausted patience, become the real story. The journey toward her grimoire is a cover for what actually matters: watching someone learn to care, and someone learn to be cared for.

Zero is a lighthearted witch who knows little about the world. She hires a beastfallen, a cursed human with the look and strength of an animal whom she simply calls Mercenary, to protect her. Together with Albus, a young magician, they set out to find the Grimoire of Zero, a spell book with power enough to destroy the world. Humans and witches are locked in endless war, and in this era the craft of witchcraft is barely understood. Beastfallen are feared by both sides and often earn their keep hunting witches, which makes the pairing an unlikely one. The Mercenary, who longs to become human, slowly reveals a kind heart as he comes to feel compassion for humans and witches alike, and the group grows closer along the way. The result is a fantasy adventure that mixes action and drama.