福尔摩斯和华生来到加拿大的魁北克出席加拿大皇家上流社会的秘密会议。然而,他们发现自己被卷入了一系列可怕的谋杀案中。当地居民传说沼泽地里有个神秘的红唇妖怪出没,但是福尔摩斯怀疑是真凶假扮妖怪吓人,阻挠大家调查真相。他判断真凶可能是村庄子里的某一个人,于是他设下了一个大胆的圈套,他把自己变成凶手的下一个目标…While attending a conference in Quebec City, Sherlock Holmes and his good friend Dr. Watson are drawn into a murder investigation in the nearby village of La Mort Rouge. Holmes had received a letter from Lady Penrose asking for his assistance as she feared for her life. It was too late however as she had already been killed by the time he received it. Her throat was torn out and the local villagers are spreading rumors about monsters and evil spirits as being the cause. Holmes doesn't believe any of that and sets out to find the killer. He believes that Lady Penrose's past as an actress may have something to with her death. As others in the village are attacked, Holmes believes the killer is among them, impersonating a local villager as he goes about his business.