A village in Canada has an old legend about a phantom that years ago killed three villagers. Many residents are afraid that the monster has come back, because some of them have seen strange luminous shapes, and then some sheep were found with their throats slashed. Their worst fears seem to be confirmed when Lady Penrose is found killed in the same way. Meanwhile, Sherlock Holmes and Dr. Watson are in Quebec, attending a meeting of the Royal Canadian Occult Society, where they are involved in a sharp debate with Lord Penrose about his village's legend. Not long after Lord Penrose receives word of his wife's death, Holmes receives a letter that the dead woman had written before she died, begging for his help and protection. When Holmes goes to the village to investigate, he finds himself hunting a determined murderer who is also a master of disguise.