Opening scene - images of a hydrogen bomb testWe see a night view of a rain swept street in Tokyo. A police officer approaches a parked car and questions the occupant (Uchida) before moving on. A second man(Misaki) appears and begins to load something into the back of the car. Misaki suddenly screams in pain and begins firing his gun. Panicked, Uchida drives off leaving Misaki struggling in the middle of the street before being hit by a taxi. The driver of the taxi jumps out to investigate. Instead of a body all he finds is a complete set of clothes laying in the street.The next day at the local police station, narcotics are found among the missing mans belongings. We are told that Misaki got the drugs from a locker rented by Mr Chin. Chin identifies the man who gave him the drugs. It is Misaki. The police go to Misakis apartment to arrest him but only find his wife, Chikako, a night club singer. Chikako agrees to go with the police, but tells them nothing, explaining she has not seen him for days, and had no idea what he did for a living. Ultimately the police dont believe her, but let her go in the hope of drawing Misaki out of hiding.While performing at the club a strange man makes contact with Chickako, he wants information about Misaki. The police arrest the man who turns out to be Dr Masada, a highly respected scientist. He explains he is doing research into the after effects of atomic explosions, he has a theory that the night Misaki disappeared, the rain may have been radioactive and Misaki actually dissolved. He also thinks that Misaki may have been on a Japanese ship that strayed too close to an atomic test conducted on Bikini or Christmas Island.Arriving home that night Chikako is attacked by a gangster from a rival gang looking for Misaki. Chikako watches the gangster leave through an open window. She screams and faints as we hear a cut of scream and a number of gunshots. Investigating, the police find yet another complete set of clothes but no body. The police remain unconvinced that Chikako is innocent, but let her remain free all the same.Dr Masada explains to the police that he has heard stories before of men dissolving, and that he has eyewitnesses to such an event. The witnesses - two fishermen relate a story of encountering a ship abandoned at sea. They board the vessel with a group of 5 other men. The ship appears in good order however during their explorations they discover no evidence of the crew other than piles of abandoned clothes While investigating the captains cabin, one of the fishermen remains behind to steal some of the clothes. A watery mass erupts out of the darkness dissolving him, leaving only his clothes. The remaining men attempt to escape as more of the watery masses appear.Only two make it back to their original ship. The Captain does not believe their story till he and the rest of the crew see strange green shapes moving around on the strange vessel. Reaching port they tell the authorities of the strange encounter and hand in a log book they picked up. Masada has the log book and reads a number of entries that seem to confirm his theory about the atomic blast being able to melt people.Masada then shows the police an experiment he has been working on that seems to show intense radiation can melt objects. He exposes a frog to the radiation which dissolves as predicted. Masada further demonstrates that despite its current state, the frog is still capable of killing things and absorbing them Chikako is convinced after what she saw that Misaki is dead, killed in similar fashion to the fishermen. The pieces come together as Masada realizes part of the abandoned ship have washed into Tokyo harbor, and the creatures themselves must be loose in Tokyo.Chikako admits she has had enough of the police harassment and offers to point out the main gangster at the club she sings at. The police surround the building then begin sending plain clothes officers inside. Some of the criminals realize something is wrong and attempt to leave, only to be rounded up by the officers.Two gangsters escape through the clubs dressing rooms, only to be cornered and killed by the creatures. Another creature appears in Chikakos dressing room and throughout the rest of the building gunfire can be heard as more of the creatures, now labeled H-men begin go to invade the property.Masada arrives at the club just as the H-men begin to leave. The police search the building and account for all the gangsters either dead or alive. Mistakenly they think Uchida has also died, however he simply removed his clothes and escaped into the night.Dr Maki (Masada's boss) shows the police a second experiment with the frog that proves the creatures are not simply killing their victims but eating them as well. Maki releases his findings to press as well as admitting the existence of the H-men. Plans are drawn up to use a combination of electricity and fire to destroy the creatures that are believed to be living now in the sewers under the city.Chikako is kidnapped by Uchida and the couple get caught up in a car chase when Masada realizes the situation. Masadas car crashes and the couple speed off. Abandoning the car, Uchida leads Chikako to a hidden stash of heroine in the sewers, which is very close to the lair of the H-men. As the couple return to the surface they see some of the H-men. Uchida orders Chakako to remove her clothes in the hope the police will think she has also been killed.Masada, figures out where Chikako might be when he finds her clothes close to a sewer exit in the river. He begins working his way through the tunnels racing against time as preparations are nearly completed by the authorities to begin their assault.Uchida now lost in the tunnels is attacked and killed by the H-men. Chikako now on her own becomes more desperate as she sees the burning gasoline released from the surface work its way through the tunnels. Masada finally hears Chikako and rescues her as she is caught between the flames and the H-men. He leads her back to a group who are following the flames through the tunnel. Their job is to destroy any remaining H-men. Ultimately the H-men are trapped and destroyed by the flames.Back on the surface Dr Maki confidently claims the H men have all been destroyed but warns if man continues to explode H bombs around the world. The H men might ultimately rule the world.