Widowed Jean-Francois Mercier is a spy posing as a military atttache in the French embassy in Warsaw in 1937. Leading a double life he may escort League of Nations lawyer Anna Skarbek,girlfriend of Russian emigre Maxim Mostov to a lavish function one night but next day he will be liaising with agent Edvard Uhl,an engineer in a tank factory who brings him blue-prints for a new German tank. Then at night he will lead a covert band investigating secret German tank trials in the forest. The Germans are suspicious of Uhl but he dies before they can arrest him. They do ...