The world's deadliest assassin, Zane, uncovers that his family was killed by the Night Owl leader. After years of planning, he takes revenge and, unburdened, dies, wishing for a life with family. Reborn in the Sonn Empire as nine-year-old Zeno, he lives with his sister, Zena, East Capital Office's constable. Soon, a murder occurs on their street. Zeno defies mockery, solves the case with keen reasoning, and exposes the killer's dual identity. Later, the Mayor was suddenly murdered. Under pressure, Zeno cracks the case again, earning the title of Sonn Empire's Greatest Detective.