In this late entry in Monogram's Jimmy Wakely series of musical Westerns, Wakely and sidekick Cannonball (Dub Taylor) come to the aid of Reno Browne, whose brother, Steven (Riley Hill), is convicted and sentenced to death for killing an enemy of their later father. Wakely and Cannonball, who has enrolled in a detective correspondence course, do a bit of investigating and are soon on the track of the real killer, Carson (Dennis Moore), the leader of a gang smuggling ore from Mexico. Although ostensibly a Jimmy Wakely vehicle, Across the Rio Grande became something of a showcase for supporting actors Riley Hill and Dub Taylor. The latter made much of his mail-order detective schtick, to the point of firing a bullet into a recently acquired "bulletproof" vest. The bullet, alas, was not deflected by the vest but by Cannonball's correspondence-manual. Across the Rio Grande marked the screen debut of radio songstress Polly Bergen, who appeared as a cantina singer under her real name Polly Burgin.