At home in England, Darling is a mischievous but much loved pet to Robert and Katherine. But when the British military asks families to volunteer their dogs to help the war effort, they send Darling off to be trained, even though it is very hard to say goodbye. Darling goes through training along with many other dogs and is ultimately used as a mercy dog, seeking out injured soldiers on the battlefield and leading the medics to them. After saving the lives of numerous soldiers, Darling is faced with a major challenge.

Murphy escapes his brutal owner and finds a new home and family in Gold Rush Alaska, where he and Sally and Mama are determined to build a new life together. Although Sally and Mama are now free from their wealthy but oppressive family in San Francisco, they find that life in a mining town is challenging, even with Murphy by their side. Sally grows to love Nome, despite the threats from wild animals, menacing humans, and harsh weather. So when it seems they may have to return to San Francisco, she and Murphy strike out on their own, hoping to find gold and make a permanent home. But danger awaits, in the form of blizzards, bears and Murphy's original owner, who will stop at nothing to get what he wants, whether it is an ill-gotten fortune or a valuable dog.