Chan, in Paris for a reunion with friends from World War I, becomes involved in investigating the murder of a munitions manufacturer who was supplying arms to the enemy, even as the rising clouds of World War II force the city into nightly blackout status..

A man involved in a crime (Nolan) kills his key witness by mistake and resigns himself to death. He changes his name so as not to harm his family. The law is not content with his explanation, however.

A team of two-fisted insurance investigators (one of whom disguises himself as The Masked Marvel) endeavor to discover and thwart the loathsome saboteur Sakima.