A new criminal has begun operating in the steamy shroud that covers Steam City. The city police cannot keep up with him- he claims that his megamaton has teleport abilities, and it seems to be true. Whenever the cops get close, the machine vanishes in a mysterious fog.

Enter the Narutaki Detective Agency. Child sleuth Narutaki hits the set, and quickly gets a hunch regarding the "teleporting" mecha. While the police chase shadows, Narutaki finds the real villain, and the battle is on. After Narutaki's megamaton, Goriki, defeats the enemy machine, the criminal reveals himself- he is Knight Phantom. He speaks of his hatred for Narutaki, and vows to destroy everything he loves. But Narutaki doesn't seem to know who this stranger is.

A major convention of world-renowned scientists is coming to Steam City, but there is a problem- someone has been kidnapping the attendees. Narutaki narrows down the list of potential targets to the next two, and goes to warn them. Later, one of them is indeed snatched, but Narutaki and Goriki foil the attempt. Later, at the convention, it is revealed that the other, Dr. Gerhalt Von Halt, is the one behind the kidnappings. He wants the knowledge of the scientists to make his megamaton better than human. He claims his evil identity- Dr. Guilty- and tries to capture the remaining scientists. Narutaki is having none of it, however, and the battle is on. Dr. Guilty's megamaton is strong, however, and Goriki is badly damaged. Just when it seems as though the heroes will be defeated, the scientist Narutaki saved earlier gives Narutaki special bullets; with which the bad guys are at last defeated.