On a rainy night in the outskirts of Tokyo, a drug smuggler, Misaki, is killed while trying to escape in a getaway car, leaving only his clothes behind. The police go to his apartment to investigate, questioning his girlfriend, Arai Chikako, who says Misaki hasn't returned home for five days. [Arai Chikako] is a singer at a cabaret, and meets Dr. Masada there. She gives him a note to take to Misaki, but it is confiscated and he is taken into custody by the police. He theorizes to the police that Misaki's disappearance is the result of his physical form melting away due to exposure to radiation in the rain that night. The police dismiss the theory.
That night, Nishiyama of a drug smuggling gang sneaks into Arai's apartment and threatens her, asking where Misaki is. However, Arai doesn't know, and Nishiyama leaves by the window, followed by gunshots. Arai screams and the police investigate, looking outside Arai's bedroom window and finding only a pile of clothes and a gun on the floor.
In the morning, the police take Arai in for questioning, but get no new information from her. [Dr. Masada] arrives at the police station and invites Inspector Tominaga and Detective Sakata to his medical institute to hear the testimony of a group of fishermen, who allegedly witnessed some of their crew members fall victim to a liquid creature, disintegrating them and leaving their clothes behind. [Dr. Masada] next shows the detectives the test effects of radiation poisoning the fishermen were exposed to on a frog. The frog melts almost immediately, all of its cells transforming into a liquid creature.
Arai later visits Masada and his superior, Dr. Maki, at the medical institute and tells him she also witnessed a man dissolving, leaving his clothes behind. Dr. Maki dubs this liquid creature the H-Man. They go to the police station again with their findings and Arai's testimony, where she also agrees to lead police to the gang at the cabaret. That night, the police go to the cabaret and make arrests, but Uchida, one of the gangsters, is tipped off by the gangster waiter and retreats to dancer Emi's room. Once inside, the gangsters try to escape through the window, but the H-Man appears, dissolving the waiter and Emi, and later Detective Sakata.
Having witnessed the carnage, Inspector Tominaga and others devise a plan to use a high voltage discharge unit to stop the H-Men's infiltration upstream by lighting Tokyo's sewer system on fire. Meanwhile, Uchida has kidnapped Arai and taken her into the sewers to retrieve the stash of drugs. Masada finds a piece of Arai's clothing floating in the water and rushes into the sewers to rescue her. Uchida is killed by an H-Man, and Arai is rescued by Masada. Both get out of the sewer in time as the flames burn all the H-Men, ending their reign of terror.
