Undercover government agent John Martin (John Wayne) has been assigned to protect the men building a road through Rainbow Valley, but construction progress is continually stalled by Rogers (Leroy Mason), the town's leading citizen and Martin's rival for the affections of local postmistress Eleanor (Lucile Browne). Rogers wants the gold-rich land for himself, so he dupes the governor into paroling hired gun Butch Galt (Jay Wilsey), who recognizes Martin from a previous encounter in prison. Believing the covert lawman is in league with Rogers and his gang of bandits, Galt furnishes Martin with dynamite in order to permanently disrupt the roadwork. With the help of mail carrier George Hale (George "Gabby" Hayes), the explosives are just what Martin needs to complete the highway, and put an end to Rogers' greedy plans.
An undercover lawman (Wayne) pretends to be an escaped convict to catch a gang of outlaws.