Buck, a powerful Saint Bernard-Shepherd dog, lives a comfortable life in the Santa Clara Valley with his owner, Judge Miller. One day, Manuel, the Judge's gardener's assistant, steals Buck and sells him in order to pay a gambling debt. Buck is then shipped to the "man in the red sweater" to be broken. Then Buck is shipped to Alaska and sold to a pair of French Canadians named François and Perrault (for $300), who were impressed with his physique. They train him as a sled dog, and he quickly learns how to survive the cold winter nights and the pack society by observing his teammates. He and the vicious, quarrelsome lead dog, Spitz, develop a rivalry. Buck eventually bests Spitz in a major fight, and after Spitz is defeated, the other dogs close in, killing him. Buck then becomes the leader of the team.
Eventually, Buck is sold to a man named Charles, his wife, Mercedes, and her brother, Hal, who know nothing about sledding nor surviving in the Alaskan wilderness. They struggle to control the sled and ignore warnings not to travel during the spring melt. They first overfeed the dogs, then when their food supply starts running out, they do not feed them at all. As they journey on, they run into John Thornton, an experienced outdoorsman who notices that all of the sled dogs are in terrible shape from the ill treatment of their handlers. Thornton warns the trio against crossing the river, but they refuse to listen and order Buck to move on. Exhausted, starving, and sensing the danger ahead, Buck refuses and continues to lay unmoving in the snow. After being beaten by Hal, Thornton recognizes him as a remarkable dog and is disgusted by the driver's beating of the dog. Thornton cuts him free from his traces and tells the trio he's keeping him, much to Hal's displeasure. After some argument, the trio leaves and tries to cross the river, but as Thornton warned, the ice gives way and the three fall into the river along with the neglected dogs and sled.
As Thornton nurses Buck back to health, Buck comes to love him and grows devoted to him. Buck saves Thornton when the man falls into a river. Thornton then takes him on trips to pan for gold. During one such trip, a man makes a wager with Thornton over Buck's strength and devotion. Buck wins the bet by breaking a half-ton sled out of the frozen ground, then pulling it 100 yards by himself, winning over a thousand dollars in gold dust. Thornton and his friends return to their camp and continue their search for gold, while Buck begins exploring the wilderness around them and begins socializing with a wolf from a local pack. One night, he returns from a short hunt to find his beloved master and the others in the camp have been killed by a group of Yeehat Indians. Buck eventually kills the Indians to avenge Thornton. After realizing his old life is a thing of the past, Buck follows the wolf into the forest and answers the call of the wild.