The story opens with Buck, a large and powerful St. Bernard-Scotch Shepherd, living happily in California's Santa Clara Valley as the pet of Judge Miller. He is stolen by the gardener's assistant, however, sold to fund the latter's gambling addiction, and shipped to Seattle. Put in a crate, he is starved and ill-treated. When released, he attacks the "man in the red sweater" but is badly beaten and taught to respect the "law of the club". Buck is then sold to a pair of French-Canadian dispatchers from the Canadian government, Fran ois and Perrault, who take him with them to the Klondike region of Canada. There, they train him as a sled dog. From his teammates, he quickly learns to survive cold winter nights and the pack society. A rivalry develops between Buck and the vicious, quarrelsome lead dog, Spitz. Buck eventually beats Spitz in a fight "to the death". Spitz is killed by the pack after his defeat by Buck, and Buck eventually becomes the leader of the team.
The team is then sold to a "Scottish half-breed" man working the mail service. The dogs must carry heavy loads to the mining areas, and the journeys they made were tiresome and long. One of the team, a morose husky named Dave, becomes sick and eventually has to be shot to end his misery.
Buck's next owners are a trio of stampeders (Hal, Charles, and a woman named Mercedes from the United States), who are inexperienced at surviving in the Northern wilderness. They struggle to control the sled and ignore warnings that the spring melt poses dangers. They overfeed the dogs and then starve them when the food runs out. On their journey they meet John Thornton, an experienced outdoorsman, who notices the dogs have been poorly treated and are in a weakened condition. He warns the trio against crossing the river, but they ignore his advice and order Buck to move on. Exhausted, starving, and sensing the danger ahead, Buck refuses and continues to lie unmoving in the snow. After Buck is beaten by Hal, Thornton recognizes him to be a remarkable dog. Disgusted by the driver's treatment of Buck, Thornton hits Hal with the butt of his axe, cuts Buck free from his traces, and tells the trio he is 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 breaks, and the three fall into the river and drown, along with the sled and neglected dogs.
Buck comes to love and grow devoted to Thornton as he nurses him back to health. He saves Thornton when the man falls into a river. After Thornton takes him on trips to pan for gold, a bonanza king (someone who hit it rich in a certain area) wagers Thornton on the dog's strength and devotion. Buck wins by breaking a half-ton (1,000-pound (450 kg)) sled free of the frozen ground, pulling it 100 yards (91 m) and winning US$1,600 in gold dust. A king of the Skookum Benches offers a large sum to buy Buck, but Thornton has grown fond of him and declines. While Thornton and his friends continue their search for gold, Buck explores the wilderness and socializes with a timber wolf from a local pack. One night, he returns from a long hunt to find that his beloved master and the others in the camp have been killed by a group of Yeehat natives. Buck eventually kills the natives to avenge Thornton, and he then is attacked by an entire pack of wolves. Buck wins the fight, then finds that the same timber wolf he had socialized with is in the pack he fought. Buck then follows the wolf and its pack into the forest, and answers the call of the wild. At the end of the story, Buck returns each year as the Ghost Dog of the Northland Legend, to mourn at the site of Thornton's death.
Output: What did Buck have to break to win the wager?