Reluctantly, after an illness, Holmes takes a holiday in Cornwall, with Watson for company. Then one day he finds himself investigating a mysterious death... after a quiet evening at home playing cards, three local inhabitants have been found in the morning in a terrible way: the two brothers overtaken by sudden mental illness, babbling incoherently, and their sister dead, with her face twisted into a look of terror. Holmes is delighted with the chance to bury his holiday boredom by taking on the case.