Ellery Richards, racing magnate, informs Mark MacArthur, head of the State Racing Commission detectives, that a ring of bookmakers and gangsters are in town betting heavily on the Richards' entry, Red Roger, and getting tremendous odds because the horse isn't conceded a chance. Richards also shows Mark that the horse occupying Red Roger's stall is a ringer. After questioning bookmaker Soapy Connor, Mark learns that the boss gambler is Larry Mann. He tells Mann that he intends to put an end to the plan by "scratching" the fake horse. Mann assures him he will not do that because he has kidnapped Mark's secretary Betty Lindale. Mark says he will back off, but secretly continues the investigation with the help of his aide, Joe Lavery. They learn, through a cast of the fake horse's bite marks, that the horse is really one named "Billy G" that belongs to a reputable owner named Weston. With the race day drawing near, and suspecting that Mann, whom he knows is an ex-police officer, has some hold on Weston, he arrests Franey Forrester, a Mann henchman, then visits Weston to tell him there is a warrant out for his arrest also. Weston, admitting his role, tells Mark that Mann is blackmailing him because he once killed a woman in a car accident. Mark places Weston under police guard and then confronts Mann. He tell him that Weston and Forrester have skipped town. Beaten, Mann offers to release Betty, for his own freedom.