At the heart of the story is the journey of Buck, the dog who is kidnapped, sold, and made to run in grueling Canadian dog races. Buck suffers many trials along the way; being separated from his master, being beaten and starved, and treated cruelly by most of the humans he is unfortunate enough to interact with.
That all changes when Thornton comes into Buck's life. Thornton, seeing how badly Buck is mistreated by three stampeders, rescues Buck and the two embark on the next chapter