Bill Boyd and Russell Hayden are sent by Bar 20 owner with some prize breeding bulls to Argentine rancher Pedro DeCordoba and it wouldn't be a Hoppy picture if he didn't aid somebody in distress.
That someone is DeCordoba who's had a string of tragedies. His son was accidentally shot while cleaning his revolver and a daughter was killed in a horse riding accident. Nobody thinks anything of it until Hoppy's down on the ranch.
The villain is unmasked rather early in the proceedings and its DeCordoba's American son-in-law Sidney Blackmer. In a story line taken from Richard III, Blackmer is quietly eliminating all the other heirs to the ranch. Now all that's left is DeCordoba and his grandson, little Joseph LaSavio.
Blackmer's a real slime ball as big a one as Laurence Olivier was in Richard III. He's got Steffi Duna on the side, the kind of girl you bed and don't wed. Not that she's not got visions of herself as mistress of a great estate. But that's not in Blackmer's plans.
Of course this film was shot nowhere near Argentina. The ranch hands are Argentine gauchos, skilled with the bola. But Blackmer's imported a bunch of American cowboys for his dirty work who must have looked real out of place on the pampas.
Sidney Toler is DeCordoba's foreman, but he hangs out with Boyd and Hayden as a third sidekick for most of the film. He certainly taught Hoppy skill with the bola because Hoppy uses it quite effectively in the climax with the bad guys.
Law of the Pampas was possibly a bit too ambitious an undertaking for a B western even if it is a Hopalong Cassidy product. But I'm sure William Shakespeare would never have dreamed one of his plays would turn up as the inspiration for a Hopalong Cassidy film.