When a beautiful woman claims that her dear husband has disappeared the investigation takes Sherlock Holmes (Robert Stephens) and Dr. Watson (Colin Blakely) to Scotland where - to their surprise - they uncover a plot involving clandestine society Her Majesty's Secret Service... and the Loch Ness Monster! But before he can deduce matters to the elementary Holmes makes an error that may jeopardize the national safety of Britain... and ruin his reputation!