Based on

Movies like Rio Grande Ranger (1936)

Sayres and his outlaw gang operate out of a town just across the border and out of the jurisdiction of the Texas Rangers. Ranger Bob Allen is sent across the border where he poses as an outlaw hoping to lure the gang back into Texas. He gets into Sayres' gang displacing the gang boss but the disgruntled ex-boss is able to expose the hoax and Bob is made a prisoner.