Buck's first owner is Judge Miller. The offspring of a St. Bernard who had been "the Judge's inseparable companion", Buck lives an idyllic life during his youth on a California ranch (Ch.1).
Buck is then stolen from the Judge by Manuel, one of his ranch hands. Manuel, in order to pay his gambling debts, sells the dog on the black market to two French-Canadian mail couriers, Perrault and Francois. Perrault and Francois are humane owners, but make a couple of extremely grueling runs with their team. After returning from the final run, they sell the team to a Scotch half-breed, who also works the mail. Buck learns the laws of survival while working on the delivery trails, and eventually vanquishes the lead dog on the team and demands to take his place.
The team is next sold to Hal, Mercedes, and Charles, who plan to use the dogs to take part in the Gold Rush in the Klondike. The trio is inexperienced and cruel, and eventually meet their demise when they fall into a semi-frozen lake after disregarding warnings that it would be dangerous to proceed upon it. Buck is saved just before the fatal run by John Thornton, who becomes his final owner.
John Thornton is "the ideal master" (Ch.VI). Buck serves him with unquestioning devotion until Thornton is killed by Indians. After Thornton's death, Buck submits competely to the call of his wild ancestors, and leaves the world of man to run with the wolves.