Hiram Brighton, the racing commissioner, hires detective Michael Shayne to watch his daughter Phyllis, a compulsive gambler, while he is away in New York. Shayne is also offered a job by Larry Kincaid to collect money from Harry Grange, a gambling racketeer who welched on a bet, but Shayne declines his offer. Soon afterward, Shayne comes face to face with Grange when he finds him feeding Phyllis money to play the roulette wheel. Shayne visits Grange's boss, Benny Gordon, the owner of the casino, and threatens to expose his crooked operation unless he returns Phyllis' losses. Shayne then escorts Phyllis home, but she returns to the casino, where she meets Elliott Thomas, the owner of the race horse Banjo Boy. Grange's continued attentions to Phyllis makes Gordon's daughter Marsha jealous, even though her father has forbidden her to see Grange. Deciding to teach Phyllis a lesson, Shayne stages Grange's death, but his plan backfires when Grange is found murdered and police captain Peter Painter suspects Shayne of the crime. Phyllis and her aunt Olivia insist on helping Shayne solve the crime, and when Phyllis finds Marsha's brooch in the car where Grange's body was discovered, Shayne pays Marsha a visit and learns that she has disappeared. Shayne then visits Gordon, where he overhears him arranging to meet Thomas at his beach house. Shayne drives to the house, followed by Phyllis, Aunt Olivia and Painter, who still suspects Shayne of murder. At the house, Phyllis is attacked by a crazed Marsha, whom she locks in the closet. Shayne then tricks Gordon into confessing that Marsha witnessed Thomas kill Grange. Shayne solves the case by explaining that Thomas killed Grange because Grange knew he had secretly substituted a horse for Banjo Boy in the big race and was blackmailing him about it.