Uh-Oh, Hoppy’s gone bad! Then again….
An outlaw named The Fox is rustling cattle on both sides of the American-Mexican border. Colonel Gonzales of the Mexican Secret Service conspires with Major Stafford of the Texas Rangers to bring him down. Stafford has Hopalong Cassidy’s (William Boyd) young pal Johnny Nelson go out to the Bar K Ranch where Hoppy is the foreman and arrest him. Hoppy goes along figuring it’ll all work out.
Stafford and Gonzales tell Hoppy the plan is to have him convince everyone he’s into some shady deals. He agrees and changes his personality and is hostile to Johnny, his good buddy Windy (George “Gabby” Hayes) and everyone else. He’s one bad tempered hombre.
Hoppy goes to El Rio where Gonzales is disguised as a street singer and stays in contact with Hoppy. He stays in a rooming house run by Grace Rand. Hoppy is not exactly polite and even tells her young daughter Molly that he doesn’t like kids. The only one Hoppy is almost civil to is local dimwit Loco.
Hoppy has a run in with Tom Parker in the saloon. Hoppy makes him back down and Tom promises they’ll settle later. He doesn’t know that Parker is The Fox’s Lieutenant. That ends quickly when The Fox is angry that Parker didn’t follow orders. He shoots him dead.
Johnny rides out to report on missing cattle. He gets bushwhacked and Hoppy takes him to town and dumps him in front of Grace’s place before he can be spotted. Johnny was out like a light and doesn’t remember anything.
It’s time for Hoppy to get serious and run down The Fox and maybe get his reputation back.