While taking a stroll in Tokyo, Linn Suzuki discovers a strange device buried in the ground. She is suddenly attacked by assailants but rescued by an enigmatic, handsome stranger who quickly defeats them. Linn had fled from Deep Kyoto to escape her abusive guardian and journeyed to Edo in search of her missing brother, who is rumored to work at the Kuon Detective Agency. The stranger is revealed to be Miho Kuon, who invites Linn to stay with her and gives her the nickname Kotetsu, a diminutive of Tetsu, after Miho's sibling Tetsujin. As events progress, a secret organization called the Syndicate begins pursuing Miho, drawing Linn further into their dangerous chase.