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. Bob and the horse elude the posse and meet up with Carlos. Carlos wounds Bob, whose day gets worse as the movie moves along,but "Black King" knocks down Carlos and is about to trample him when the sheriff arrives, arrests Carlos and frees Bob, whose innocence has been established.