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..

An inebriated mouse with a throbbing head takes a priceless diamond, thinking it's a soothing piece of ice. Two policemen, one of them a lunkhead, are assigned to recover the missing jewel.