The Gold Rush in the Yukon and Alaska has prompted the need for big, strong dogs who will be able to pull sleds over icy trails. Buck, a large animal living in the home of Judge Miller in Santa Clara, California, is exactly what the explorers want. He is an animal with human-like tendencies, intelligence, strength and dignity. Thus far he has enjoyed his civilized life with the occasional nature stroll or hunting trip. Manuel, a gardener's helper with a penchant for gambling and a need for money, manages to kidnap Buck and sell him on the black market. He is given to a saloon-keeper and transported via train to the Northland. Throughout the ordeal, Buck is kept in a cage and becomes increasingly angry. He manages to badly bite one of his kidnappers. By the time he arrives at his destination, he has worked himself into a rage.
The meeting with the Red Sweater and the painful encounters with his club push Buck into submission. He is not broken, but he knows better than to keep resisting, which can only result in his death. Once he is behaving correctly, Buck, along with Dave and Curly, two other dogs, is sold to Francois and Perrault, two agents of the Canadian government. They must bring the mail between Skaquay and Dawson, Alaska. While in camp, Curly is killed for trying to make friendly advances to another husky. Buck understands that this is the law of club and fang which dominates this new world. He resolves never to go down in that manner.
Buck and Dave join a preexisting dog team led by Spitz, a bullying husky dog. The trail work begins immediately. Buck learns fast from the other dogs. In time he starts to become more wild, losing the domesticity imposed upon him in the Judge's home. Tensions develop between him and Spitz. Buck is ready to be a leader, and looks to usurp Spitz's power. While hunting a rabbit one night, the two end up in a fight to the death. Buck achieves mastery. When Francois attempts to place Sol-lek at the head of the team, Buck refuses to allow it. Even the appearance of a club does not faze him, for he has learned how to avoid it. When Francois allows him to take the position, he is amazed by Buck's abilities. Buck begins to dream of an ancient world in which man and dog fought side by side to survive.
After two round trips between towns, the dogs are exhausted and overworked. Francois and Perrault are very proud of their team, having just set a record for their run. But they receive new orders and must bid the dogs goodby. Several Scottish "half-breeds" take charge of Buck and his other dogs. He does not have strong feelings for them, but they are good men and they care for the dogs. Buck's dreams of the ancient world grow more vivid. The path is very difficult and men and dogs alike are growing weak. One day Dave shows so much pain that one of the men tries to get him to run behind the sled. He refuses so adamantly that the men give in and allow him to run himself out. The next morning they drive the sled away, then one man returns and shoots Dave.
After this last trip, the Scottish men are told to sell the dogs and buy fresher ones. The team is sold to a group of tenderfoots -- Hal, Charles, and Mercedes -- looking to strike it rich. They have no idea how to work with a dog team. In Hal's eagerness to complete the trail, he terribly mistreats the dogs. His quiet brother-in-law Charles and weepy sister Mercedes only increase the difficulty. Hal's incompetent handling of the rations leads them to run out of food for the dogs half-way through the trip. Starving and overworked, one by one the dogs start to die. The remnants pull into the camp of John Thornton. Buck refuses to rise when Hal wants to leave. After watching the cruel young man beat Buck repeatedly, John steps in and saves him by cutting him out of the harness. The rest of the team continues. Only a quarter-away, the sled falls through the ice, thinned by the sun, and dogs and humans perish together.
Buck falls wildly in love with John Thornton, who immediately recognizes that Buck is one of a kind. Under John's influence, comforted by his two other dogs Skeet and Nig, Buck begins to heal. He accomplishes a number of miraculous things for John, saving his life twice and winning a bet that allows him to pay off all his debts. He does not forget his visions of the primitive world, but he is happy at John's side. Along with Hans and Pete, John's partners, the dogs go on an expedition for a lost mine. The work on the trail, the daily hunting, are absolutely delightful for Buck. Though they don't find the mind, they do find gold, and so there is no more work for the dogs to do. Buck ruminates once more on the call that he hears nightly in the forest.
Eventually he starts to sleep away from the camp. He embraces his instincts and wild tendencies, killing his own food and watching out for himself. Buck meets a wolf who befriends him and is quite sad when he returns to the camp. This pattern continues, until one day Buck returns to the camp to find everyone killed by the Yeehats, a Native American tribe. He flies into a rage at the death of beloved John, and kills all the men who do not run away from him. There is no more tie to mankind, so Buck returns to the forest and remains with a pack of wolves. Each year he visits to the valley where John Thornton died, mourning his lost, dearest friend.