An intrepid member of the Royal Canadian Mounted Police confronts a band of masked rustlers who ambush stagecoaches in the rugged borderlands and terrorize local settlers. After a near-fatal exchange on a storm-swept gorge, he becomes embroiled in a series of pursuits, ambushes, and rescues around a fortified lakeside homestead called Water Castle and a timber ark, defending inhabitants against repeated sieges. The narrative follows tracking and countermeasures, clues that illuminate the gang’s methods, and culminating actions to round up the white-hooded raiders, including a daring final confrontation that resolves the threat to the community.