Chinese sleuth Charlie Chan discovers a scheme for the theft of government radar plans while investigating several murders.

Sadie Martin owns a riverboat that is frequently used by miners traveling to their claims. During their trip, the miners drink and gamble. Sadie's daughter, Helen, is unaware of her mother's work because her mother sends her to boarding school in order to live a lifestyle more attributed to the upper-class. Unfortunately for Sadie, she is facing difficulty maintaining the costly riverboat. She is soon forced to sell the boat in order to make ends meet.