Los Angeles homicide police lieutenant Dick Chasen recalls a perplexing case he recently investigated, which began the day before convict Charles “The Butcher” Benton’s execution: Benton’s attorney, Paul Lowe, visits his client on death row in San Quentin Prison to inform him that the governor has turned down their appeal. When Benton accuses Lowe of failing him, Lowe tells Benton that he worked against himself by hording the $600,000 payroll he stole from an armored car, and double-crossing his partners, Squeamy Ellis and Joe Marcellia, were freed after turning state’s evidence against him. As Benton is due to die the next day, Lowe, who was the mastermind behind the robbery, scoffs at his client's threat to kill him, Ellis and Marcellia. Following the execution, Prof. Bradshaw, a San Francisco biochemist, prepares for the final stages of an experiment he hopes will lead to a cure for cancer. Bradshaw applies 287,000 volts of electricity to a dead body that his assistant has purchased from the morgue, unaware it is Benton. Bradshaw is stunned when Benton revives, but the electricity has altered Benton's cellular structure so that his flesh is now impenetrable. Benton, obsessed with his pledge to kill his former partners, overpowers and murders Bradshaw and his assistant, then heads for Los Angeles on foot. Although Benton's case is officially closed, Dick still hopes to recover the stolen money, and invites Benton’s former girl friend, burlesque dancer Eva Martin, on a date. While they share a hamburger dinner in Dick’s car, Eva, who originally came to Los Angeles for a screen test that failed, reveals that she was never Benton’s girl friend; she had only befriended him after her roommate left him. She then explains that after the robbery, police assumed they were having a relationship. Dick also watches Lowe, Marcellia and Ellis. One night, Lowe meets with Marcellia and hires him for a new heist. Benton, meanwhile, leaves a trail of bodies in his path to Los Angeles: after murdering a man from whom he steals a car, he kills two policemen who attempt to stop him at a roadblock. Dick and his captain, J. L. Lauder, read newspaper accounts that bullets cannot stop the “indestructible” killer, but doubt the sanity of the witnesses. Nevertheless, Lauder assigns the case to Dick, as the then-unidentified marauder appears to be headed for Los Angeles. However, Benton is already in the city and immediately goes to see Eva. Although he is now mute, Benton proves his identity to her with a tattoo. When he sees that an envelope containing a map that he had sent her before his death is empty, Eva reveals she showed the map to Lowe. After Benton storms out Eva tries to alert Dick, but he is out of the office, so she leaves a message. She then calls Ellis to warn him about Benton. Ellis escapes his hotel apartment in time, but Marcellia is unable to avoid Benton, who murders both him and a policeman. Benton then goes to Lowe’s office, which is empty. However, when Ellis arrives, Benton throws him to his death from the fifth floor railing. Eva takes refuge at Dick’s office, where he and Lauder receive reports about the murders of Bradshaw and his assistant. In addition, they learn that a morgue attendant has admitted to selling Benton’s body. Lauder is incredulous that their killer may be a dead man, despite Eva’s assurances of Benton’s identity. After yet another random murder, a nervous Lowe insists on seeing Dick and Lauder. When they refuse to provide police protection, Lowe strikes a sergeant so that he will be incarcerated. However, the policemen trick Lowe into believing that they will not press charges. Fearing that he will be released and vulnerable to attack by Benton, Lowe confesses to having hired Benton, Ellis and Marcellia to rob the armored car, and reveals where Benton hid the payroll. Most importantly, Lowe reveals that Benton was able to elude capture by using the city sewer system. As the police post men at various storm drain entrances, Benton returns to the sewer system to retrieve his cache. Dick and Lauder are assisted by uniformed police, one of whom is armed with a flamethrower, as they track Benton in the sewers. Benton retrieves his money, but is caught in the act by Lauder. When Benton tries to escape, the police fire on him with the flamethrower. Benton is horribly burned but escapes to an electric power station, where he then purposely electrocutes himself to death. Dick now concludes his police report, adding that the stolen money was recovered. Lauder gives Dick some time off, which Dick uses to propose marriage to Eva, with whom he has fallen in love. Eva accepts, now recovered from the shock of the bizarre events.