Alexandria Carr and her brother Nicky have traveled to the Yukon and seek Sergeant Preston's help to trace a 25 carat emerald jewel that has been stolen from them by a man who they believe is called Louie Millano. Millano has covered the emerald in gold to disguise it as a gold nugget and intends to sell it on to the Gates a bonanza Millionaire in the Yukon, Gates. However Millano is killed by Cragman, Gates manager and the nugget stolen. This is not the only surprise in the story.