Both Sprague and Jett and their crews are hunting buffalo. Doan is with Sprague and is looking for the Jett outfit where his girlfriend Milly is being held against her will.
The "trailer" is a dog named Caesar and the killer is a mountain lion, aka as a cougar or puma the narrator was quick to point out.
Burgess and Greeley are rustling horses and shooting Indians. When they kill Manual they frame Lieutenant Allister. His older brother John now attempts to defend him at his murder trial.