The true identity of Nightlight Mask, who killed the Duke of Le Mond and stole his opal, is Lupin! Working on the deduction of novelist Conan Dorill, the mystery man will appear at the costume ball, kill the daughter of Count Johannes and steal a second opal. An innocent Lupin is not at all satisfied. Professor Dolmen has the third opal. How will the revenge concealed within the ancient treasure of the Incas resolve itself?