Synopsis
In this film, much of the main plot has already happened or happens off-camera. We see a subplot that out-of-work private detective Mike Shayne is hired by his friend, wealthy racing executive Hiram Brighton, to, while he's out of town, watch over his spirited daughter Phyllis, who gambles her money away and has begun dating underworld character Harry Grange, to the consternation of the father.
The key plot involves wealthy but shady Elliott Thomas who owned a horse, Banjo Boy. The odds were 15 to 1 against that horse winning. Thomas had a run of bad luck and needed a quick infusion of cash. In south America he found a champion to switch with the same markings as, and a dead-ringer for, Banjo Boy. So as not to affect the track odds, Thomas engaged Grange as a shill to "spread around" Thomas' large bet of $10,000. The substitute horse wins, and the payoff is $150,000 (equivalent to over $2 million in 2021 purchasing power).
But Grange would not pay Thomas his money, so Thomas hired another shady character, Larry Kincaid, who tries to hire Shayne to go to Shayne's friend and Grange's boss, casino owner Benny Gordon, to ask him to pressure Grange. Shayne refused, so Kincaid went straight to Grange. When Grange told Kincaid the backstory, Kincaid decided to cut himself in on the pot by blackmailing Thomas.
As a result, Thomas and Kincaid brawled, Kincaid ended up dead, and Grange knew of the meeting. At that point, Thomas felt he had no choice but to do away with Grange.
Shortly before Thomas kills Grange, after a meeting in Gordon's casino, Grange had been drugged, then driven to the woods, smeared with ketchup and left in the convertible by Shayne in a scheme to teach Phyllis a lesson about hanging with shady characters. The scheme backfires when they find Grange has actually been shot dead and Shayne's gun is on the ground, having been fired. Shayne has already tipped the police to "a murder in the woods" in the plan to scare Phyllis. Now sirens are heard. Shayne rushes Phillis away in her car but his car won't start and cops arrest him.
In reality, Gordon's daughter Marsha, Grange's angry ex who had been jilted by Grange, happened to stumble upon the crime scene first, became distraught and rushed to her father. In order to protect her from any kind of suspicion, Gordon framed Shayne, having conveniently seen him leave the casino earlier with Grange.
Eventually, Shayne correctly deduces what happened with the murder of Grange, and engineers a trap for Thomas in front of the police, at which point Thomas confesses the horse race plot, the death of Kincaid and throwing his body in the bay, and the murder of Grange, at which point the movie ends.
