An operative from the Wells Fargo company goes undercover to trap a crooked sheriff and his equally nefarious hirelings in this standard B-Western from A.W. Hackel's low-budget Supreme Pictures Corp. Former gridiron star (and onetime leading man to Greta Garbo!) Johnny Mack Brown plays Steve McLain, the witness to a stage holdup. Investigating, Steve learns that crooked Sheriff Pegg (Horace Murphy) is in cahoots with the local saloon proprietor Ace Pringle (Ted Adams). Their nefarious scheme: to rob the Wells Fargo, for whom, unbeknownst to the sheriff, Steve is working . In an effort to outsmart the criminals, Steve is ambushed by a couple of Pringle's henchmen and left to drown in the river. Our hero, however, is rescued in the nick of time by lovely Linda Forbes (Suzanne Kaaren) and survives to put a permanent halt to Sheriff Pegg's criminal activities. Undercover Man was the first of Hackel's Mack Brown westerns to be distributed by newcomer Republic Pictures.