Buck is a big, strong dog who is part St. Bernard and part Scotch shepherd. In the beginning of The Call of the Wild, Buck is domesticated and lives on a California estate owned by Judge Miller. Buck sees himself as the prince of the estate, ruling over the domain. The Judge and his offspring treat Buck kindly and respectfully. Then Buck is captured and shipped to the Northland to work as a sled dog. There, he learns the brutal "law of club and fang," and realizes he must kill or be killed, eat or be eaten. As Buck adapts to life as a sled dog, he connects with his primordial survival instincts. Eventually, Buck takes over leadership of the dog team after defeating the former leader, Spitz. Buck undergoes another change when John Thornton saves him from three abusive sled guides. As Thornton nurses Buck back to health, the dog forms a strong bond of love with him. However, when Buck leads Thornton's dogsled team into the remote wilderness, Buck's primordial instincts develop even more, and he reaches his full potential as a fierce predator. When Thornton is killed, Buck breaks all links with the human community and becomes the leader of a wolf pack, thereby fully answering the call of the wild.
John Thornton is an outdoorsman and a prospector who lives in the Northland with his two friends, Hans and Pete. Thornton has a close connection to the wilderness and to dogs. He has no fear of the wild because he knows how to survive in it. He shows his great capacity for love when he nurtures Buck back to health. Like many of his companions in the North, Thornton is prone to drinking and bragging. During one such episode, he bets Buck can pull a sled loaded with 1,000 pounds of flour, but he immediately regrets doing so. However, the devoted Buck wins the bet for him. Also like many Northland people, Thornton dreams of attaining riches by mining gold. Because of this, he and his friends go into a dangerous territory, where they find abundant gold but end up dead.
Spitz is a big snow-white dog who leads the dog team Buck joins. Spitz can appear friendly on the surface, but underneath he is cunning and sneaky, and he's a fierce, wolflike fighter. Spitz uses his fighting prowess to discipline members of his dog team. After seeing Spitz take pleasure in watching a dog being killed by a pack, Buck develops a hatred for Spitz. Spitz feels threatened by Buck. Most dogs from the Southland can't survive in the wild, but Buck is powerful and shows he can adapt and learn. Spitz tries to pick a fight with Buck, but at first Buck avoids confrontation. Later, though, Buck shows he is unafraid of Spitz, and Spitz gets really angry. Eventually, Spitz confronts Buck and they have a fierce battle; Buck cripples Spitz, who is then killed by the dog team.
Along with his sister, Mercedes, and her husband, Charles, Hal buys a dog team that includes Buck. Hal is a callow young man who constantly needs to prove his toughness and masculinity. He wears a Colt revolver and a hunting knife strapped to a belt loaded with cartridges, but Hal knows little about guiding a dogsled or surviving in the wild, and he refuses to learn. He also treats the dogs brutally, beating the exhausted animals when they don't perform as well as he expects. Because of his ignorance and stubbornness, Hal remains oblivious to the danger in which he puts himself, his companions, and his dogs. When Hal beats Buck, John Thornton intercedes and saves the dog.
Mercedes, her brother, Hal, and her husband, Charles, buy a dog team that includes Buck. Like her brother and husband, Mercedes is shallow and inept. At first she hates to see the dogs being beaten, but when the going gets rough on the trail, she no longer shows compassion for the animals. In fact, she insists on riding in the sled even though the dogs are starving and exhausted. Mercedes sees herself as a refined lady who deserves constant pampering; spoiled and immature, she ends up bickering with her companions like a bratty child. Mercedes is too obsessed with her petty desires to realize the danger she and her companions are in.