Chief Inspector Wakabayashi of the Tokyo Police is confronted with a seemingly impossible murder which took place aboard an airliner. It may be linked to a recent wave of unsolved killings. Meanwhile, his old friend Dr. Tsukioka, who's working with famous physicist Prof. Hayakawa and engaged to Hayakawa's daughter, Akiko, has accidentally discovered the secret of invisibility in the course of his cosmic ray research. A clue left at a bank robbery in which the thief somehow got past locked doors and barred windows without a sign of a break-in, stabbed a guard and made off with several million yen in cash leads Wakabayashi and his men to the Club Asia. Its owner, Kuroki, denies any involvement with criminals. Two more stabbings follow: First, a secretary who works for successful businessman Kusunoki, and then the bartender at the Club Asia. In both cases, no assailant was seen, but the buzzing of a fly was heard. The attacks are being committed by a man named Yamada, who works for Kusunoki. The businessman possesses a gas which can shrink a man to the size of a fly, which Yamada uses to get close enough to his victims to strike without warning and get away. The gas is also an addictive drug. Kusunoki wants vengeance against those who let him take the fall for war crimes. Loose cannon Yamada the Human Fly is also insanely jealous of Sexy Dancer Mieko, who performs at the Club Asia, which is why he killed the bartender and the secretary, and then murders the nightclub's owner. After Yamada kills a detective who was tailing him, and then the dancer Mieko, Wakabayashi begs Tsukioka to turn him invisible, believing it's the only way he'll apprehend the Human Fly. Tsukioka refuses, because every attempt so far to reverse the process kills the subject. Kusunoki learns of the invisibility ray and sends Yamada to steal it. Entering the lab through a vent, the Human Fly kills Prof. Hayakawa and his assistant, Sugimoto, but forgets to take the ray. After the professor's funeral, Tsukioka decides to turn the invisibility ray on himself. Slipping into Kusunoki's office, he listens to the crime boss ordering Yamada to make another attempt to steal the ray. The Human Fly accidentally kills himself at the lab: He flies too close to a beaker, is overcome by fumes and plunges headfirst into the corrosive chemical. Wakabayashi attempts to arrest Kusunoki. The businessman claims he's innocent, until invisible Dr. Tsukioka speaks up and tells the police he overheard Kusunoki and Yamada plotting. The businessman uses the shrinking gas to escape. To force Tsukioka to give him the invisibility ray, Kusunoki sets off a bomb on a railroad overpass, killing hundreds. He threatens to detonate another bomb in a week if Tsukioka won't surrender the ray. As the deadline nears, Kusunoki sends a letter to the press instructing Wakabayashi to meet him at midnight, with the ray, atop a tall building. Kusunoki flies in by helicopter. He almost succeeds in stealing the invisibility ray machine. But Tsukioka's fiancé, Akiko, has also turned herself invisible. She hides in the helicopter and forces Kusunoki to return at gunpoint. Kusunoki tries to shoot it out with the police. After receiving multiple gunshot wounds, Kusunoki falls to his death. Tsukioka turns the invisibility ray over to the government.