Inventor Henri Giffard, working on the creation of a new engine for an airship, receives anonymous threatening letters that are dropped at his door by a lady in black. What prompted the lady of noble birth to take such an inappropriate step? Perhaps Madame Fleur's past hides a terrible secret. Henri's friend, the aristocrat and adventure-lover Viscount de Moreau, undertakes to unravel this charade. But the matter is complicated by the fact that he is irresistibly attracted to the mysterious beauty. Literary adaptation by O. Koltsova.