This is quite an engaging little mystery. Sir C. Aubrey Smith ("Collison") is director of the National Art Museum charged with repatriating artworks after the end of WWII. When some colleagues from the Parisian Louvre arrive to collect the "Mona Lisa", all seems fine. Fine, that is, until the slightly dodgy art collector "Hoffmeyer" (Erich von Stroheim - with a full head of hair!) arrives at his home next day with the painting - and points out that it is a fake! A bit of investigation soon realises that it is, indeed, a forgery - but where is the real one? The mystery itself is all a bit lightw...