On a Los Angeles street, Officer Hollis, a patrolman on his way home from work, stops a man he suspects of being a burglar and is shot and mortally wounded. The minor clues lead nowhere. Two police detectives, Sergeants Marty Brennan (Brady) and Chuck Jones (Cardwell), are assigned to catch the killer, Roy Morgan (Basehart), a brilliant mystery man with no known criminal past, who is hiding in a Hollywood bungalow and listening to police calls on his custom radio in an attempt to avoid capture. His only relationship is with his little dog.
Roy consigns burgled electronic equipment to Paul Reeves (Whit Bissell), and on his fifth sale is nearly caught when he shows up to collect on his property. Reeves tells police that the suspect is a mystery man named Roy Martin. The case crosses the paths of Brennan and Jones, who stake out Reeves’ office to arrest and question Roy. He suspects a trap, however, and in a brief shootout shoots and paralyzes Jones. Jones wounds Roy, who performs surgery on himself to remove the bullet and avoid going to a hospital, where his gunshot wound would be reported to the police.
With his knowledge of police procedures, Roy changes his modus operandi and becomes an armed robber. During one robbery he fires his semi-automatic pistol, and the police recover the ejected casing. Lee (Jack Webb), a forensics specialist, matches the ejector marks on the casing to those recovered in the killing of Officer Rawlins and the wounding of Sgt. Jones, connecting all three shootings to one suspect.
Captain Breen (Roy Roberts) uses this break to gather all of the witnesses to the robberies. They assist Lee in building a composite photo of the killer. Reeves then identifies Roy from the composite. However, Roy hides in Reeves’ car and attempts to intimidate him into revealing details of the police investigation. He barely eludes a stakeout of Reeves’ house.
Because the police do not realize that Roy has inside knowledge of their work, the case goes nowhere. Breen takes Brennan off the case in an attempt to shake him up. Jones convinces his partner to stop viewing the case personally and to use his head.
Plodding, methodical follow-up by Brennan, using the composite photograph, results in information that Roy, whose actual name is Roy Morgan, worked for a local police department as a civilian radio dispatcher before being drafted into the Army. Brennan tracks him down through post office mail carriers and disguises himself as a milkman to get a close look at Morgan and his apartment.
The police surround and raid the apartment that night, but Morgan, forewarned by the barking of his dog, escapes through the attic and uses the Los Angeles sewer system as a means of escape. The film continues with a dragnet and chase through the sewers. Roy is finally cornered by the police in a passage blocked by the wheel of a police car. As the police shoot tear gas at Roy, he staggers and attempts to fire at them. He is then shot down and killed. The final scene is notable for its resemblance to the final scene in The Third Man in which Orson Welles is chased through the sewers of Vienna. No known connection between the films has been established.