Late one night, on a Los Angeles city street, a plainclothes policeman is murdered in front of a young couple. When it is learned that the murdered man, Ed Monigan, was robbed of $1,000, police captain Forster suspects that Monigan was on the take and involved in an illicit gambling ring that is operating out of a nearby cigar store, and that his murder is somehow connected to a new betting operation that is trying to take over all the bookies in the city. When police detective Mike Conovan is summoned to the crime scene, he disputes the captain's theory, and is certain that Monigan was not involved in any illegal activities. The captain assigns Mike to the case, along with veteran detective Fred Piper and a young rookie nicknamed C. C. The three detectives begin their investigation, with their only clues coming from the eyewitnesses, who described the killer as being left-handed and having a mottled face. In the hope of learning more about the killer, Mike questions a police informant named Sleeper about Arthur Webson, a known gambling ring operator, and any new associates Webson may have acquired. Though the interrogation provides no new leads, C. C. finds a matchbook in Sleeper's possession, in which the names Turk Kingby and Lafe Douque are inscribed. After adding Kingby and Douque to the list of possible suspects or accomplices, Mike pays a friendly visit to Monigan's son Ed. Ed angrily blames Mike for his father's death, claiming that he would not have placed himself in such a dangerous position had he not been trying to prove himself to Mike. Ed then asks Mike to devote himself to clearing his father's good name. While searching for Kingby, Mike and C. C. witness two men forcing a man into a car. They follow the car to a warehouse, which is guarded by Umpire Menafoe, a former distillery operator and head of the bookie operations in the city. Menafoe, an old friend of Mike, allows Mike to watch a group of bookies line up suspects thought to be involved in a series of holdups. Later, Sleeper tells Mike that Kingby and Douque are part of a gang known as the Royalty Boys, and that they served time in Wallaby Prison. When Mike and Piper visit a private investigator named Pontiac, he tells them that Kingby had been associating with a "sizzler" named Lili, who was a burlesque dancer at the Club Fol-de-Rol. Mike visits Lili at the club and cultivates her trust by taking her to the movies and spending time with her. Later, Mike's wife Gloria makes a failed attempt to convince her husband to quit his dangerous work and take a safer job at a steel plant. After secretly planting a hidden microphone on a gangster named Hippo, Mike and some other detectives listen in on a conversation in which Douque confesses to killing a police officer. Douque, however, is killed shortly thereafter. When Hippo tells Mike that Kingby is planning to make a "big hit" against the bookie organization that killed Douque, Captain Forster sets out to snare the killers. Before the criminals are captured, however, Piper succumbs to a trap set by the killers and dies. Realizing that Lili must have tipped off the hitmen, Mike goes to the nightclub and demands an explanation. Lili confesses that she loves Kingby and that she set up all the murders to protect him. When Kingby is finally captured, Mike discovers that he used a special rubber glove to make himself appear to be left-handed, and that he used shoe polish to mottle his face. With the case solved, Mike is finally able to devote his attentions to Gloria.