Some years before, a vicious criminal and master of disguise known as `The Fox` is captured by Rurales Colonel Gonzalez and Hoppy working undercover. When `The Fox` escapes from prison, he vows to exact vengeance from the two lawmen. He lures Cassidy south of the border with a forged letter from Gonzalez`s father and murders Gonzalez in cold blood. Aiding `The Fox` in his plans is his sister Janet, to whom the chivalrous Hoppy finds himself attracted.