In Jack London's hard-edged and naturalistic adventure, Buck is first described as a large dog of one hundred and forty pounds, sired by a St. Bernard with a mother who was a Scotch shepherd dog. "He had a fine pride in himself, was ever a trifle egotistical." But, Buck is not a pampered dog since he hunts, loves the water, and other outdoor activities.
After he is stolen and sold, Buck is beaten by a man with a club and he learns that such a man is to be obeyed because he has the power to kill. Buck is new to the ways of wolves, however, but after he witnesses Curly be ripped apart by them, he learns that they work together. Another dog, Spitz, becomes Buck's adversary.
As he lives with the other sled dogs and learns to pull a sled, Buck's feet are too tender, but one of the men fashions moccasins for him until his feet toughen. Buck begins to enjoy pulling a sled and learns to burrow into the snow at night in order to stay warm. An intelligent dog, he learns from observation of the others and finds that he can dominate some of them easily. Having adapted well, Buck has a capacity to adjust himself to situations. He is strong and does not back down from a fight; Buck is an Alpha dog. Not only does he adapt, but instincts "long dead came alive" in Buck.
In vague ways he remembered back to the youth of the breed, to the time the wild dogs ranged in packs through the primeval forest, and killed their meat as they ran it down. It was no task for him to learn to fight with cut and slash and the quick wolf snap. In this manner had fought forgotten ancestors. They quickened the old life within him, and the old tricks which they had stamped into the heredity of the breed were his tricks.
The dominant primordial beast was strong in Buck, and under the fierce conditions of trail life it grew and grew.
And, yet Buck becomes very loyal to his last owner, Thorton, even saving his life. When Thorton disappears, Buck follows his trail and finds his body riddled with arrows. Filled with rage, he attacks the Indians who killed Thorton. Then, Buck becomes brother to others and joins wild wolf packs. But, once a year he returns to the site of Thorton's death.