Buck is the undisputed leader of all the dogs on Judge Miller’s estate in California. A crossbreed of St. Bernard and Scottish shepherd, he inherited the size of the first and the intelligence of the latter. Buck cannot know that the lust for gold hit the human beings of the country and that dogs of his breed are much in demand as sled dogs in the frozen North. Consequently, he is not suspicious when a workman on the estate takes him for a walk one night. The man takes Buck to the railroad station, where the dog hears the exchange of money. Then a rope is placed around his neck. When he struggles to get loose, the rope draws so tight that it shuts off his breath, and he loses consciousness.
He recovers in a baggage car. When the train reaches Seattle, Washington, Buck tries to break out of his cage while he is being unloaded. A man in a red shirt hits him with a club until he is senseless. After that, Buck knows that he can never win a fight against a club. He retains that knowledge for future use.
Buck is put in a pen with other dogs of his type. Each day, some of the dogs go away with strange men who come with money. One day, Buck is sold. Two French Canadians buy him and some other dogs and take them on board a ship sailing for Alaska. The men are fair, though harsh, masters, and Buck respects them. Life on the ship is not particularly enjoyable, but it is a paradise compared to what awaits Buck when the ship reaches Alaska. There he finds men and dogs to be little more than savages, with no law but the law of force. The dogs fight like wolves, and when one is downed, the pack moves in for the kill. Buck watches one of his shipmates being torn to pieces after he loses a fight, and he never forgets the way one dog in particular, Spitz, watches sly-eyed as the loser is slashed to ribbons. Spitz is Buck’s enemy from that time on.
Buck and the other dogs are harnessed to sleds on which the two French Canadians carry mail to prospectors in remote regions. It is a new kind of life to Buck but not an unpleasant one. The men treat the dogs well, and Buck is intelligent enough to learn quickly those things that make him a good sled dog. He learns to dig under the snow for a warm place to sleep and to keep the traces clear and thus make pulling easier. When he is hungry, he steals food. The instincts of his ancestors come to life in him as the sled goes farther and farther north. In some vague manner, he senses the great cunning of the wolves who have been his ancestors in the wilderness.
Buck’s muscles grow firm and taut and his strength greater than ever. Yet his feet become sore, and he has to have moccasins. Occasionally, one of the dogs dies or is killed in a fight, and one female goes mad. The dogs no longer work as a team, and the two men are on guard constantly to prevent fights. One day Buck sees his chance; he attacks Spitz, the lead dog on the sled, and kills him. After that, Buck refuses to be harnessed until he is given the lead position. He proves his worth by whipping the rebellious dogs into shape, and he becomes the best lead dog that the men have ever seen. The sled makes record runs, and Buck is soon famous.
When they reach Skaguay, the two French Canadians have official orders to turn the team over to a Scottish half-breed. The sled is heavier and the weather bad on the trip back to Dawson. At night, Buck lies by the fire and dreams of his wild ancestors. He seems to hear a faraway call like a wolf’s cry. After two days’ rest in Dawson, the team starts back over the long trail to Skaguay. The dogs are almost exhausted. Some die and have to be replaced. When the team arrives again in Skaguay, the dogs expect to rest, but three days later, they are sold to two men and a woman who know nothing about dogs or sledding conditions in the northern wilderness. Buck and the other dogs start out again, so weary that it is an effort to move. Again and again, the gallant dogs stumble and fall and lie still until the sting of a whip brings them to their feet for a few miles. At last, even Buck gives up. The sled stops at the cabin of John Thornton, and when the men and the woman are ready to leave, Buck refuses to get up. One of the men beats Buck with a club and would have killed him, but Thornton intervenes, knocking the man down and ordering him and his companions to leave. They leave Buck with Thornton.
As Thornton nurses Buck back to health, a feeling of love and respect grows between them. When Thornton’s partners return to the cabin, they understand this affection and do not attempt to use Buck for any of their heavy work. Twice, Buck saves Thornton’s life and is glad that he can repay his friend. In Dawson, Buck wins more than a thousand dollars for Thornton on a wager, when the dog breaks loose a sled carrying a thousand-pound load from the ice. With the money won on the wager, Thornton and his partners go on a gold-hunting expedition. They travel far into eastern Alaska, where they find a stream yellow with gold. In his primitive mind, Buck begins to see a hairy man who hunts with a club. He hears the howling of the wolves. Sometimes he wanders off for three or four days at a time, but he always goes back to Thornton. At one time, he makes friends with a wolf that seems like a brother to Buck.
Once Buck chases and kills a great bull moose. On his way back to the camp, he senses that something is wrong. He finds several dogs lying dead along the trail. When he reaches the camp, he sees Indians dancing around the bodies of the dogs and Thornton’s two partners. He follows Thornton’s trail to the river, where he finds the body of his friend full of arrows. Buck is filled with such a rage that he attacks the band of Indians, killing some and scattering the others.
His last tie with humanity broken, he joins his brothers in the wild wolf packs. The Indians think him a ghost dog, for they seldom see more than his shadow, so quickly does he move. Had the Indians watched carefully, however, they could see him closely. Once each year, Buck returns to the river where Thornton died. There the dog stands on the bank and howls, one long, piercing cry that is the tribute of a savage beast to his human friend.