A bored Sherlock Holmes (Robert Stephens) meets Madame Petrova (Tamara Toumanova), a famed ballerina, who tries to seduce him, hoping that their child will have her body and his brains. He manages to extract himself from her grasp, using Dr. Watson (Colin Blakely) as a decoy. Then he and the doctor head for Loch Ness in search of the missing husband of Gabrielle Valladon (Genevieve Page). While there, they meet Sherlock's brother, Mycroft (Christopher Lee), who aids them in solving the mystery.