Japan is one of seven nations selected to send a man (or woman) into space for the latest expedition on the moon. Participants include the United States of America (woman), United Kingdom (man), Germany (woman), Norway (man), Russia (man), South Korea (man), and of course Japan (man, one of two primary characters, named Ichiro Fukuda). New expanses of the moon are explored and there are vast samples collected by the members. Though they report only the good, the Japanese crew member has been feeling a little odd or "off" since he collected his first soil sample. The project is considered a wild success for the wealth of information that will hopefully be gained, and the H-III returns to Earth.
The Japanese astronaut's minor illness progresses to a moderate stage wherein he inquires about it upon the physical checkup (required for an astronaut when they return to Earth). The doctor runs a couple of tests and finds nothing immediate but takes a sample to test for strep throat; the immediate sample shows no sign but as we know it can manifest overnight. Fukuda is processed and greets his eager wife, Miki Fukuda. As she takes care of him over the course of a week, he notices his feet started to mutate in shape and discolor. He buys larger shoes and hides this from his wife, because as the weeks pile on it appears that it is spreading. The doctors are unable to help him and he resigns himself to a strange death. And yet, he doesn't die. He feels changes inside his body but any semblance of feeling ill per se is gone. One morning he awakens and realizes it has progressed to the point where it is creeping down his shoulders. His wife will found out soon enough. In despair, he steals away to the forest and beyond.
Fukuda survives in the forest on wildlife, but he misses his wife dearly. Unbeknownst to him, his wife is aware that he has not been well. She suspects something mentally with Fukuda has gone awry. She notifies the police of his disappearance and searches around for him to no avail. At this time, Fukuda's transformation is complete, or so he thinks. He sees his reflection in the water as some freakish alien life form. At this point he begins to grow exponentially. His food supply is not always sufficient in the forest and he compensates by searching for food around dumpsters near his hometown, etc. Reports start flooding in about a creature eating anything living aside from humans. This piques the interest of Miki, who begins investigating the reports and interviews people who claim to have seen the creature. Any encounter with humans, they claim, he is gentle, though they still worry about the creature. She walks into her bedroom one evening and sees the alien looking at the picture stand next to the window, intently studying a picture of Ichiro (himself) and Miki. He sees her out of the corner of his eye and runs away at full speed. He loses her quickly but she becomes convinced it is her lost husband. She is labeled mentally deranged and begins being ignored by the towns people.
As time goes on, Fukuda becomes too large to easily hide anymore (he is roughly 15 feet tall and remains bipedal but can maneuver on all fours when necessary). He discovers the changes in his body mean he no longer requires oxygen. Hiding in the ocean is the next best thing to disappearing. Fukuda has a grizzly encounter with a large shark and a bloody battle ensues. His new found size, strength, and exoskeleton-reminiscent structure give him enormous potential for physical battle. As a result, he wins. Battered and bruised, the ocean doesn't seem so safe anymore. He takes back to the lakes and rivers of the forest, hiding in the water and around it. Unfortunately, scientists and the military have taken interest in this mysterious creature reported to have been around the small city. Miki pleads with them that the creature is her husband. They brush off her outlandish allegations outwardly; in secret a few members believe the story does make a modicum of sense. At the very least, the pieces all fit. They are interested in what unprecedented events could unfold if they captured an alien form, or alienized human in this case. Sure enough, while his guard is down they find Fukuda.
Gut wrenching and painful experiments are applied to Fukuda, who has long since stopped being able to speak any form of human language. Their intent is not at all to be inhumane, but they cast caring for him aside as they experiment as much as possible. They notice he continues to grow and begin to draw up plans to move him somewhere else (he has grown to an enormous 35 meters tall). They do not accurately calculate his strength increases, though, and he breaks out. Standard weaponry is useless, and Fukuda realizes he can spit at long distance a gooey, sticky acid. He levels the military personnel with relative ease, but when a pair of high-power helicopters show up he retreats into the forest for cover. The government cannot keep what's occurring in the forest hidden for very long and quickly it's breaking news around Japan. Miki hurriedly drives to the area but has to continue on foot when she's stopped by guards keeping the area clear of citizens. Back in pursuit, the choppers fire on Fukuda, who is hit by some stray bullets and winces in pain. Angrily he jumps and rips down one of the choppers to the ground. The other lands several direct shots and he jumps into a lake. The helicopter fires all over the water randomly in hopes of killing him. Some dark fluid rises to the surface and they report they've mortally wounded the creature. As they slowly lower the chopper over the center of the lake the water parts with the blades of the chopper blowing them around. Out of nowhere Fukuda's hand jumps up and flings the helicopter right onto land. Fukuda rises out of the water and walks over to the two pilots, injured by attempting to crawl away. He picks up one and gruesomely impales him with his sharp, almost talon like claws. He starts to eat the soldier, who is still partially alive, when he feels gunfire on him. He turns around to the second pilot shooting him with a machine gun. The poor pilot is unaware that such weaponry is useless. Fukuda drops the now dead pilot and walks over to this one, biting his head off.
As he swallows the head he sees a woman near the forest edge staring at him. It's Miki. Her horrified face snaps Fukuda back to planet Earth (pardon the pun) and he looks down at the headless pilot in his head, then over at the smashed helicopter and the first dead soldier's remains. Miki starts crying and runs away. That's no longer the Ichiro Fukudo she knew and loved. Fukuda is horrified at what's happened, but at this time several more helicopters and ground troops show up. Fukuda stares the missile launchers straight down their barrels and allows himself to be blown into a million pieces. Fukuda lost any will to live. He didn't even have himself anymore. The military begins cleaning up the mess now that the chaos is over. The end of the movie shows Miki laying in bed. Covered up almost completely by the covers, she adjusts and turns over, her shirt showing part of her shoulder, which is displaying the same discolored and disfigured look that afflicted Ichiro. She's been infected too and now she's seen what it does.Artwork for creature is the work of Toho Kingdom.
Artwork for people is the work of Toho.