Don Carlos, Mexican renegade, wanted by his government for espionage, but who poses as a horse buyer,wants to buy "Black King," Owner Jim Riddle refuses and Carlos is determined to get the horse by hook or crook. A race is arranged between a Carlos horse and "Black King", with the winning owner to take the other's horse. Carlos sends a henchie to fix "Black King" so that he goes lame and loses. Bob Horton, who loves Riddle's niece, Helen, rides the horse and is accused of pulling a double-cross. That night at Riddle's gambling house Carlos loses, and shoots a gambler with Bob's gun.Bob is accused and escapes to Mexico. Exhausted, he finds a water hole and, while drinking (and surprised to find that it isn't poisoned, considering the kind of day he has had) ol' "Black King" (the Horse With a Human Brain) limps up. He has been freed by Betty, Helen's six-year-old sister. Bob discovers the wire that had been twisted into the horses' leg by the henchman.