Fuurouta is a prank-loving ghost with transformation abilities. He became a bird at a department store in Ginza and let free a beautiful insect that was on sale. However, it was no average insect at all. The "Black Fog" that occurred at the same time flowed to Nagatacho and engulfed the area around the National Diet Building. Jiro and Kikko, who were handling the problem, meet Fuurouta at the police station and take him to the "Superhuman Bureau." Fuurouta gets interested in the bureau's activities and tries to go against the "Black Fog" in order to join...
During the opening of a department store, a ghost named Fuurouta plays a prank on a class trip, accidentally freeing a prized green beetle from its cage, as a mysterious black fog rolls through the area. Shortly after the incident, the black fog settles on the Japanese Diet building as the Superhuman Bureau restrains Fuurouta, unsure of what to do with him. While escaping from captivity, Fuurouta runs into a strange girl named Campe and offers to be friends with her. Meanwhile, Jiro finds the corpses of Japanese Public Security superhumans as they were defeated by the Tartaros Bugmen from inside the black fog. Fuurouta overhears Professor Hitoyoshi describe how he has revived an ancient virus to kill the Tartaros Bugmen, and the next day he offers to help the Bureau kill them in exchange for letting him join. After taking the virus, Fuurouta pockets the green bug from before, then tosses the virus container like a bug bomb, killing the Tartaros Bugmen and dissipating the black fog. Later, Jiro casts the deciding vote to let Fuurouta join the Bureau.
Five years later, Fuurouta is attacked by a grown-up Campe, who turned out to be the green bug he rescued from before. However, after his actions she became the last of the Tartaros Bugmen alive while the politicians who destroyed their forest habitat went unpunished. Jiro appears and talks Campe down from attacking Fuurouta. As she leaves, Fuurouta cries that the black-and-white world he thought he knew became more complicated.