Complex sentence: Young Lord John Grey, an English scout, stumbles upon Jamie and Claire; and imagining Claire a prisoner, tries to "save" her, whereupon Jamie breaks the boy's arm but spares his life.
Split into simpler sentences: Young Lord John Grey, an English scout, stumbles upon Jamie and Claire. Imagining Claire a prisoner, Grey tries to "save" her, whereupon Jamie breaks the boy's arm but spares his life.