Mike Shayne, an out-of-work private detective, is hired by his friend, the wealthy racing executive Hiram Brighton, to keep an eye on his spirited daughter, Phyllis Brighton, while he’s away. Phyllis gambles away her money and has begun dating a dangerous underworld figure, Harry Grange, much to her father’s consternation. The arrangement sets up a tense moral backdrop as Shayne tries to tread a careful line between loyalty and propriety, all while a family’s trust hangs in the balance.
The central intrigue centers on the wealthy but shady Elliott Thomas, owner of the horse Banjo Boy. With the odds stacked against Banjo Boy—15 to 1—the desperate Thomas travels to South America in search of a perfect look-alike to swap in without altering the betting line. He enlists Grange as a sham backer to “spread around” Thomas’s hefty bet of $10,000, and the ruse pays off when the substitute horse wins, yielding a staggering payoff of $150,000. The windfall should have secured Thomas’s cash flow, but it also seeds a dangerous web of blackmail, betrayal, and murder.
When Grange refuses to honor the agreement, Thomas hires Larry Kincaid to pressure him, and Kincaid seeks a closer connection to the casino owned by Benny Gordon in hopes of increasing his take. Shayne refuses to become an conduit for coercion. After Kincaid chooses to press Grange directly, the situation spirals: Grange is entangled in the scheme, and Kincaid moves to cut himself in for a larger share. A violent confrontation ensues, Kincaid dies, and Grange becomes aware of the meeting. Thomas, meanwhile, decides Grange must be eliminated to prevent exposure.
In a dramatic sequence staged just before Grange’s fate, a meeting at Gordon’s casino leads to Grange being drugged, driven into the woods, smeared with ketchup, and left in a convertible with a gunshot wound. Shayne has already tipped the police to a supposed murder in the woods as part of a plan to teach Phyllis a hard lesson about associating with shady characters. The scene grows chaotic when the car refuses to start and the police close in, arresting Shayne just as the truth begins to surface.
As the investigation unfolds, Marsha Gordon—Gordon’s daughter and Grange’s jilted ex—stumbles on the crime scene and is shaken by her father’s predicament. Gordon, seeking to shield his daughter from suspicion, frames Shayne, throwing the detective into a perilous fight to clear his name. Yet Shayne remains persistent, analyzing the threads of deceit until the pieces fit.
At the story’s end, Shayne orchestrates a final trap in front of the authorities, compelling Thomas to confess the full extent of his scheme: the horse-race substitution, Kincaid’s death, the disposal of Kincaid’s body in the bay, and the murder of Grange. The confession exposes the web of manipulation that started with a high-stakes bet and spiraled into murder, leaving Shayne vindicated and Phyllis spared from a perilous association with the underworld. The case closes with a hard-won sense of justice, a cautious reconciliation for the Brighton family, and a reminder that deceit in the world of gambling and greed can unravel a life in an instant.
