Henry Suggs, by daytime a vigilante and leading citizen of the town of Cattelo, is by night a marauder who terrorizes the countryside; his true identity is known only by Tarzan, a horse whose master is killed by the desperado. Suggs succeeds in having the horse condemned to death because of his supposed wickedness, but Ruth Gaunt, daughter of the murdered man, persuades Tom Drury, an itinerant cowboy, to find a way to save Tarzan. A permanent friendship springs up between Tom and the horse, while Tom and Suggs become rivals for the hand of Ruth. After a number of narrow escapes, Tom and Tarzan unmask the villain and ride triumphantly home with their prisoner.