Le mystérieux Monsieur Sylvain 77 years old

In an aero-naval base, a marine engineer who was about to deliver the name of a redoubtable spy is murdered. Shortly afterwards three different men appear on the crime scene, each of them claiming to be Monsieur Sylvain, the expected sleuth. But which of the three investigators is actually Sylvain? And what about this delightful young lady who arouses one of the police inspectors? Couldn't she be the murderous spy?