While on vacation in Cornwall, the brilliant consulting detective Sherlock Holmes is asked to investigate a very strange case. A woman named Brenda Tregennis has been found dead at a table in her house. Two of her brothers, George and Owen, are found seated next to her, still alive but completely insane. The dead woman and the mad men all have looks of terror on their faces. A few days later, a third brother, Mortimer Tregennis, is also found dead with the same look of horror on his face.