Tansang Loop is a space station with a population of two million, officially speaking. But really, it's thrice more, including the cyborgs and androids. Io is a Rohm Model Five, a cyborg made with a biomechanical brain and synthetic muscle. While on a routine spacewalk, Io stumbles upon the body of a murdered detective-and improbably, both Io and the detective are Model Fives. Io tries to solve the detective's last case in order to find out who killed him, but in doing so, may reveal that their lives are more interconnected than meets the eye.