Buck is living a happy life in California until he is sold to pay a gambling debt. Taken to the Klondike to become a sled dog, Buck must toughen up and learn the harsher rules of survival in the North. One of the first of these is how to deal with being harnessed in the same team as a dog that wants to kill him. Large, strong and smart, Buck toughens to his new life. But even the toughest dog can be worn down by constant work, and after 3,000 miles of pulling sleds, Buck nears the end of his rope. Cast away as no longer useful, Buck is acquired by greenhorns whose inexperience nearly kills him, but after being saved by John Thornton, he at last finds a man he can love. Then on a remote gold-hunting expedition, Buck hears a call emanating from the woods and speaking to the wild heart of his distant ancestors. The lure of it almost balances the great love he bears for Thornton, but events take him away from his old life... and into legend.