The Streets here are continually enshrouded in white mist. Steam rises from everywhere, obscuring the streets and buildings. Because coal was the only fuel available, this city developed an incredibly advanced steam technology. Unfortunately, evil-doers take advantage of this white fog to commit countless crimes and continually baffle the police. At night, the white mist shrouds the darkness, intimidating everyone. People Call this city enveloped in mist, STEAM CITY.

Merc Storia takes places in a world where humans and monsters coexist. The protagonist, Yuu, is a healer apprentice and possesses the ability to tame monsters. In a quest to regain the memories of Merc, a girl confined in a bottle, the pair embarks on a journey.

Nine super-powered cyborg soldiers join forces to combat Black Ghost, the terrorist organization which they were originally created to serve.