There was no doubt about the murder weapon. You could see the outline of the horse's hooves...
But that was the second murder. When San Francisco private investigator Nick Polo rejoins his date at the races after a rather prolonged trip to the men's room, the first thing she says is "Are you feeling all right?" And the second is "Your zipper's undone." There is reason for both. Polo is not looking too well, and quite understandably. He'd just discovered the dead body of one of his Uncle Pee-Wee's employees, a small-time hood named Johnny Aiello, in one of the men's room stalls. Enough to make anyone forget to pull up his zipper.
An even better reason for the smart, slick private eye's discomfort soon materializes: the police think that Solo, who has served some time for a less-than-egregious crime, may be Johnny's killer. So it doesn't look at all good when Nick goes out to keep an appointment with a prospective client and discovers yet another body—the client's—kicked to death by one of his own horses. It's clear the horse hadn't taken the initiative, and the cops choose to credit Polo.
The odds are against Polo; the closer he comes to the real murderer, the higher they climb.