Buck, a huge, four-year-old half-Saint Bernard and half-Scottish shepherd dog, is living a life of civilized ease in California's Santa Clara Valley in the home of Judge Miller. It seems to be the best of all possible worlds, for Buck is the most prized animal that the Judge owns. Around this time, however, gold is discovered in the great North, and large dogs suddenly become tremendously valuable because these types of dogs are needed to haul the heavy sleds through the deep snow fields.
Tragically, for Buck, one of the Judge's servants (an addictive gambler) steals Buck and sells him to a ring of thieves who are making a great deal of money by buying and selling dogs to northern traders. Buck's spirit, however, does not adapt as easily as do some of the other docile big dogs. Buck cannot tolerate being tied up and beaten; he fights against his cruel new master, but all of his efforts to escape are futile. Thus, Buck learns the new concept of "master," even though he learns it reluctantly: a man with a club is a master and must, at all costs, be obeyed.
After days of travel on both train and boat, Buck discovers that he is in the primitive North, and there he rapidly learns to conform to the laws of the primitive new world. For example, he encounters such problems as how to work as a member of a dog team pulling a sled, how to burrow into a hole in the snow in which to sleep, how to survive perpetual hunger pains, and how to rely on his native intelligence and his animal instincts.
Buck also soon learns that the dominant primordial bestial instinct is very strong in him, and he learns just as quickly that when he is attacked, he must take the offensive immediately in order to survive; as a result of this type of living adjustment, Buck also learns that he has to live a life of almost continual alertness, as well as in almost continual pain and discomfort. Yet Buck has one advantage: his size makes him fearful to the other dogs. Still, however, all is far from pleasant, for even though Buck can defend himself quite well and is ever-ready to scrap with another dog, he has a secret that he must keep to himself: Because Buck has arrived so recently from civilization, the craggy ice and snow of the North tear at his paws and make his work extremely painful.
After being in constant hunger for many days, Buck's old instinct to kill and eat raw meat and warm blood is rekindled within him. About this same time, Buck is constantly pitted against another powerful dog, Spitz, the lead dog of his sled team. After several skirmishes with Spitz, Buck's decisive fight with him occurs, and the result of the fight is a victory for Buck, who then becomes the lead dog. In his position of leadership, he quickly proves himself to be superior to all the other dogs and thus wins the admiration of his masters, François and Perrault, who work with Buck quite some time before they are called away to other duties.
Buck's next master is a Scotch half-breed; the man is fair, but he works Buck almost beyond endurance, so much so that on a difficult run against extremely adverse conditions, most of the other dogs succumb to the wild elements. Buck, however, survives, even though he loses a significant amount of weight. Buck's next change in life occurs when he and his team are sold to three amateur adventurers - Charles, Hal, and Mercedes; they have absolutely no concept of how to discipline the dogs or even how to drive a team through the frozen northern snow. As a result of their ineptitude, the dogs' food supply is gone before the trip is half over. At this point, Buck sees the futility of trying to continue; thus, he simply refuses to return to the trace (the harness) despite the fact that he is severely beaten. Propitiously, a man named John Thornton appears and threatens the three owners if they continue to beat Buck.
Buck's instinct concerning the three amateur adventurers proves to be correct; Charles, Hal, and Mercedes continue on their way across the frozen snow and ice and lose their lives, plus the lives of the rest of the dog team when they try to cross a river of melting ice. All are drowned.
