Two prospectors, a dog, and a woman find gold and overcome ruthless competitors in this greatly altered adaptation of Jack London's novel.
In the Yukon in 1900 Jack Thornton (Clark Gable) wants to go back to Seattle, but he gambles away his gold. Out of prison, Shorty Hoolihan (Jack Oakie) tells Jack he opened a letter and found a valuable map. Jack has enough credit to get an outfit, and they become partners. Jack buys a St. Bernard named Buck for $250 so that Smith (Reginald Owen) will not kill him. Buck runs off but returns during a snowstorm. Jack shoots at wolves and finds Claire Blake (Loretta Young). She wants to search for her husband, but he takes her by force. As a prisoner she refuses to help cook. They cross a river on a raft but lose their sled. They walk into Dawson, where Claire thanks Jack, and he apologizes. She has the original map, and Jack asks for her help. They need money for an outfit. Jack says Buck can pull a thousand pounds. Smith wants to kill the dog that is hostile to him, and he bets $1,000 against the dog. Jack calls Buck, and Buck pulls the load a hundred yards as men cheer.
Shorty packs their equipment. Jack tells Claire to stay at the hotel; but she argues and insists on going. The three go down river on a raft. Jack says grabbing is the law of the Klondike; but Claire believes in more humane law. Jack says Buck remembers being a wolf. They find the cabin and gold. Shorty leaves to file the claim. Jack repairs the roof as Claire cooks. Buck hears wolves and follows one, but he comes back. Jack plays with Buck. Jack and Claire work. He kisses her and wants her.
Smith has found John Blake (Frank Conroy) and demands the map; but Blake memorized it so they won't kill him. They find the cabin, and Smith has his men get rid of Blake. They walk in with guns, take the gold and all the tools and transport so they can file a claim and Jack can't. However, Smith and his two men capsize the canoe in the rapids and drown. Buck finds Blake in the snow; he revives and is reunited with his wife. Jack tells Claire he is keeping her; but she says Blake needs her. She follows her law, not Jack's. The Blakes leave and plan to return in the spring. Jack kisses Claire good-bye. Buck hears the call, and Jack lets him go back to the wild. Buck and the wolf have puppies. Shorty returns after filing the claim and brings in an Indian cook.
This movie version moves the focus from the dog to the melodrama of the people. Greed and violence prove self-destructive, while those who are on the "up-and-up" are successful. Buck returns to nature and Claire to her husband, leaving Jack with gold he grabbed by "Klondike law."