Mal Granger, a skilled telephone technician with expertise in electronics, is recruited by the notorious gangster Vince Walters to help expand his legitimate business, which secretly functions as a front for illegal bookmaking activities. Thanks to Granger's technological savvy, the operation flourishes, and he also develops a more efficient system for collecting race track information. As Granger begins to enjoy a larger cut of the profits, he demands a 20% partnership—a request Walters agrees to, consolidating Granger's rising influence in the criminal world.
However, trouble soon follows when Walters attempts to collect debts from a bookie. The bookie reacts violently, shooting Walters and then himself, throwing Granger into the chaos. Taking charge of the wire service and the illicit betting racket, Granger becomes a significant figure in the underground gambling scene, but also draws the attention of Lieutenant Pete Wright of the Los Angeles police, who begins to scrutinize him closely.
Meanwhile, East Coast mobster Larry Mason, dispatched by his boss Carl Stephans, arrives with the aim of convincing Granger to join his syndicate. During these interactions, Granger’s personal life becomes complicated as he develops an attraction to Mason’s wife Gail. As they grow closer, Granger decides to accept a 50/50 split in his operations with Mason and Stephans, but not everyone in the gambling world is pleased with this new arrangement. Several independent bookies resist paying the increased “protection” fees and are subsequently harassed or roughed up by syndicate enforcers.
Trouble escalates when Granger’s assistant, Trudy, uncovers evidence that Granger is being shortchanged in his share of the takings. When he confronts Walters’ people, he is told that the missing money is due to "necessary expenses", but Granger vows to get what he’s owed. His personal life also becomes dangerous; after Mason beats Gail, Granger hires a hitman named Gizzi to eliminate Mason with a rifle. Gizzi's decision to blackmail Granger afterward leads to a deadly confrontation at the Malibu Pier. Granger, in a desperate move to protect himself, crushes Gizzi to death against the pier’s railing with his car, then disposes of his body by tossing it into the water.
Using his expertise in telephone communications, Granger orchestrates an alibi by making a call to Lieutenant Wright while pretending to be in Palm Springs—only for police to realize he was lying when they hear a streetcar whistle, which doesn’t exist in Palm Springs. This evidence, along with matching paint from Gizzi’s damaged car, eventually ties Granger to the murder.
Convinced he needs to leave the criminal life behind, Granger plans to escape to Guatemala with Gail. Before doing so, he seeks to recover the money owed to him. With Gail and his loyal associate Chippie, he hacks into a phone line at a Las Vegas betting parlor to intercept and record race results, allowing them to place large bets on the winning horses after the results are broadcast with a short delay. However, their scheme is compromised when Chippie is recognized by a man holding a grudge, leading Carl Stephans to send the police after them.
As the authorities close in, Granger and Gail race to escape the law by heading to Boulder Dam in an effort to cross into Arizona and avoid arrest. Their escape attempt is thwarted when they encounter a police roadblock. They join a tour group and retreat into the dam itself. Exhausted, Gail collapses from fatigue, and Granger, despite his efforts to find safety, is shot. He is believed to have been killed just as he approaches the Arizona border, ending his tumultuous and dangerous life as a criminal caught between the forces of law and crime.
