In chapter five, Charles, Hal, Mercedes and their small pack of traumatized, exhausted dogs arrive at John Thornton's camp located at the mouth of White River. Hal strikes up a conversation with John Thornton, who is whittling an axe handle made of birch. John Thornton advises Hal not to cross the river because the bottom is likely to fall out and they will break through the ice. He also mentions that it was sheer luck that they made it this far and says he wouldn't attempt to cross the thin ice for all the gold in Alaska. Hal responds by calling Thornton a fool and begins to severely whip Buck, who refuses to get up. Buck senses impending doom and withstands the powerful blows as Thornton looks on.

Fortunately, John Thornton intervenes by demanding that Hal stop beating Buck. When Hal pulls out his hunting knife, Thornton strikes Hal's hand with his axe handle and refuses to let him touch the wounded dog. Shortly after Charles, Hal, and Mercedes pull away, Thornton and Buck watch as the bottom of the trail gives out. The humans and dogs fall through the broken ice and are never seen again. John Thornton saves Buck's life and becomes his new master. Unlike Buck's other masters, Thornton genuinely loves the dog. The two become inseparable.