A young mail carrier rides a hazardous mountain route between isolated settlements, repeatedly encountering a solitary, mysterious woodsman who leaves cryptic notes. When highwaymen ambush him to steal the pouch, he resists and is drawn into schemes that involve missing registered letters, traps, an arrest, a curious court hearing, perilous night rides, flood and a runaway river, and an underground chase. Through resourcefulness and courage he uncovers the conspirators' plans, reveals a long-hidden secret, and brings a surprising resolution that secures the lost correspondence and exposes the truth behind several strange events.