Imagine a world where coal is the only fuel source and steam technology is highly advanced; in this world lies a fog-covered city known as Steam City. And under the cover of the mist, evil people use megamatons – steam-powered robots – to wreak havoc on the townspeople. Narutaki, a teenage detective with a brilliant mind, is the only person who can put a stop to these villains. With the aid of the nurse Ling Ling and the megamaton Goriki, Narutaki will stop at nothing to guarantee the safety of Steam City and all of its inhabitants!