A frontier party investigates mysterious rifle reports that seem to originate beneath a canyon and discovers unusual activity around a nearby mine. Encounters with strangers, Native individuals, and outlaws produce fights, rescues, betrayals, and a sequence of clues including a letter, a ring, a locket, and picture-writing. Action moves from ore-dumps and drift tunnels to stagecoach perils and a dramatic pursuit, resolving the mine’s secrets and reuniting scattered companions.