This is a cleverly constructed thriller featuring an on form Jean Simmons as the naive girl holidaying with her brother "Johnny" (David Tomlinson) in Paris at the time of the Great Exposition. Things take quite a mysterious turn on their first morning when not only has her brother disappeared - but so, too, has his hotel room! In the best tradition of "the Lady Vanishes" (1938), nobody in the hotel - staff or guest - recalls ever having seen him and she is widely suspected of being either mad, or just a chancer hoping to avoid paying her bill... It is only the timely intervention of Honor Black...