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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!
The story takes place at a certain middle school where several murder cases take place. Kobayashi, a boy who goes to this school, meets genius detective Akechi, who comes to the school to investigate. Kobayashi takes an interest in Akechi and, in spite of his friend Hashiba's worries, he volunteers to be Akechi's assistant.