Buck slowly gets his strength back. John Thornton, it turns out, had frozen his feet during the previous winter, and he and his dogs are now waiting for the river to melt and for a raft to take them down to Dawson. With Thornton, Buck experiences love for the first time, developing a strong affection for the man who saved his life and who proves an ideal master. Thornton treats his dogs as if they are his own children, and Buck responds with adoration and obeys all commands. Once, to test Buck, Thornton tells him to jump off a cliff; Buck begins to obey before Thornton stops him.
Even though Buck is happy with Thornton, his wild instincts still remain strong, and he fights as fiercely as ever. Now, however, he fights in defense of Thornton. In Dawson, Thornton steps in to stop a fight in a bar, and one of the combatants lashes out at him. Immediately, Buck hurls himself at the man’s throat; the man narrowly escapes having his throat ripped open when he throws up his hand, though Buck succeeds in partially ripping it open with his second try. A meeting is called on the spot to decide what to do with Buck, and the miners rule that his aggressive behavior was justified, since he acted in defense of Thornton. Soon, Buck has earned a reputation throughout Alaska for loyalty and fercocity.
Buck saves Thornton’s life again when Thornton is thrown out of a boat and gets caught in fierce rapids. Buck swims to the slick rock where Thornton clings for his life, and the other men attach a rope to Buck’s neck and shoulders. After several failed attempts, Thornton grabs onto his neck, and the two are pulled back to safety.
That winter, on a strange whim, Thornton boasts that Buck can start a sled with a thousand pounds loaded on it. Other men challenge his claim, betting that Buck cannot perform that task before their eyes. A man named Matthewson, who has grown rich in the gold rush, bets a thousand dollars that Buck cannot pull his sled—which is outside, loaded with a thousand pounds of flour. Thornton himself doubts it, but he makes the bet anyway, borrowing the money from a friend to cover the wager. Several hundred men come to watch, giving odds—first two to one, then three to one when the terms of the bet are clarified—that Buck cannot break out the sled, and a confident Matthewson throws on another $600 at those odds. Once Buck is harnessed in, he first breaks the sled free of the ice, then pulls it a hundred yards. The crowd of men cheers in amazement, with even Matthewson joining in the applause.