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 and murders Gonzalez in cold blood. Aiding 'The Fox' in his plans is his sister Janet, to whom the chivalrous Hoppy finds himself attracted. (thetvdb.com)