As the title says Hoppy, Lucky, and Windy go south of the border In Old Mexico paying a visit to Al Ernest Garcia playing old friend Don Carlos Gonzalez in answer to a summons. Not like Don Carlos isn't happy to see an old friend, it's just that he didn't do any summoning.
That note was written by escaped criminal Paul Sutton a notorious outlaw known as The Fox who has escaped a hanging and looking to settle the score with Hoppy and Don Carlos's son Trevor Bardette who is a colonel in the Mexican Rurales. They put The Fox away once.
Sutton kills Bardette outright but still has Hoppy to deal with. What was unusual here for a Cassidy western is that most criminals had a healthy respect for Hoppy's instincts. Sutton has them, he just doesn't care. Sutton has a carefully laid plan to put Hoppy in his hands and it's going according to schedule or so he thinks.
In fact Betty Amann plays a femme fatale in charge of seducing Mr. Cassidy. Bill Boyd is up to her tricks as well.
It's all good in the end though as you would expect a Hopalong Cassidy picture would be.