Sun and Sky, the horse favored to win the Kentucky Derby and possibly the Triple Crown, is being shipped to Kentucky to participate in the upcoming race. During the flight, however, all four members of the cargo plane's crew are overcome by some kind of drug, though the pilot manages to radio a distress call before losing consciousness. Mannix is hired by the insurance company holding the policy on the missing horse to investigate what happened. His search turns up the missing plane, and all four unconscious men, but not Sun and Sky. Mannix discovers that at least two of the men might have had a motive to steal the horse -- and then receives a confession from the horse's owners that they, too, are having financial problems that would make them suspects as well.
—aldanoli