Dr. Watson (Nigel Bruce) has donated his services to the Musgrave estate, a makeshift wartime hospital set up by three feuding siblings. When Watson's colleague Dr. Sexton (Arthur Margetson) is mysteriously attacked, he asks his friend Detective Sherlock Holmes (Basil Rathbone) to help with the investigation. Upon arrival, Holmes discovers that one of the estate's proprietors has been murdered, and clues lead him to suspect a bizarre Musgrave family ritual as the possible cause.