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.

As Knight Phantoms reign of terror continues, Steam City has been evacuated and the city is a mere ghost town in the wake of his devastation. It's going to be Narutaki's most intense battle yet, but with so many people caring about their beloved Steam City, even enemies may be willing to lend a helping hand.