This appears to be the first of eight follow-up films to 1936's "Lightnin' Bill Carson".  Federal lawman William "Lightning Bill" Carson has a reputation for being lightning fast on the draw. He is also apparently a master of disguise. In this movie, he sets out to prove his nephew innocent of robbing a bank shipment by infiltrating the bad guy's gang under the persona of Mexican money launderer Jose Fernandez. The good guys triumph.