The French Key plot
"The Adventures of detective ace Johnny Fletcher"
Private detective Johnny Fletcher and his assistant Sam Cragg leave their rented room. But when they secretly return to retrieve some of their forgotten luggage, they discover a murdered man with a gold coin in his hand on the bed. A coin collector tells Fletcher that this coin is an old and valuable Spanish coin. Fletcher soon begins to suspect that this man himself is involved in the murder. Fletcher's investigation leads his assistant Sam to become involved in a gold smuggling operation and even a second murder.