Buck, a powerful dog, half St. Bernard and half sheepdog, lives on Judge Miller’s estate in California’s Santa Clara Valley. He leads a comfortable life there, but it comes to an end when men discover gold in the Klondike region of Canada and a great demand arises for strong dogs to pull sleds. Buck is kidnapped by a gardener on the Miller estate and sold to dog traders, who teach Buck to obey by beating him with a club and, subsequently, ship him north to the Klondike.
Arriving in the chilly North, Buck is amazed by the cruelty he sees around him. As soon as another dog from his ship, Curly, gets off the boat, a pack of huskies violently attacks and kills her. Watching her death, Buck vows never to let the same fate befall him. Buck becomes the property of Francois and Perrault, two mail carriers working for the Canadian government, and begins to adjust to life as a sled dog. He recovers the instincts of his wild ancestors: he learns to fight, scavenge for food, and sleep beneath the snow on winter nights. At the same time, he develops a fierce rivalry with Spitz, the lead dog in the team. One of their fights is broken up when a pack of wild dogs invades the camp, but Buck begins to undercut Spitz’s authority, and eventually the two dogs become involved in a major fight. Buck kills Spitz and takes his place as the lead dog.
With Buck at the head of the team, Francois and Perrault’s sled makes record time. However, the men soon turn the team over to a mail carrier who forces the dogs to carry much heavier loads. In the midst of a particularly arduous trip, one of the dogs becomes ill, and eventually the driver has to shoot him. At the end of this journey, the dogs are exhausted, and the mail carrier sells them to a group of American gold hunters—Hal, Charles, and Mercedes.
Buck’s new masters are inexperienced and out of place in the wilderness. They overload the sled, beat the dogs, and plan poorly. Halfway through their journey, they begin to run out of food. While the humans bicker, the dogs begin to starve, and the weaker animals soon die. Of an original team of fourteen, only five are still alive when they limp into John Thornton’s camp, still some distance from their destination. Thornton warns them that the ice over which they are traveling is melting and that they may fall through it. Hal dismisses these warnings and tries to get going immediately. The other dogs begin to move, but Buck refuses. When Hal begins to beat him, Thornton intervenes, knocking a knife from Hal’s hand and cutting Buck loose. Hal curses Thornton and starts the sled again, but before they have gone a quarter of a mile, the ice breaks open, swallowing both the humans and the dogs.
Thornton becomes Buck’s master, and Buck’s devotion to him is total. He saves Thornton from drowning in a river, attacks a man who tries to start a fight with Thornton in a bar, and, most remarkably, wins a $1,600 wager for his new master by pulling a sled carrying a thousand-pound load. But Buck’s love for Thornton is mixed with a growing attraction to the wild, and he feels as if he is being called away from civilization and into the wilderness. This feeling grows stronger when he accompanies Thornton and his friends in search of a lost mine hidden deep in the Canadian forest.
While the men search for gold, Buck ranges far afield, befriending wolves and hunting bears and moose. He always returns to Thornton in the end, until, one day, he comes back to camp to find that Yeehat Indians have attacked and killed his master. Buck attacks the Indians, killing several and scattering the rest, and then heads off into the wild, where he becomes the leader of a pack of wolves. He becomes a legendary figure, a Ghost Dog, fathering countless cubs and inspiring fear in the Yeehats—but every year he returns to the place where Thornton died, to mourn his master before returning to his life in the wild.