Holmes receives an odd letter that makes reference to vampires. Mr. Robert Ferguson, who comes to 221B Baker Street the next morning, has become convinced that his Peruvian second wife has been sucking their baby son's blood. By his first wife, he has a 15-year-old son named Jack, who suffered an unfortunate accident as a child and now, although he can still walk, does not have full use of his legs. Since the start of the bloodsucking, Jack has unaccountably been struck twice by his stepmother, although Mr. Ferguson cannot imagine why. Ever since being found out by her husband, she has locked herself in her room and refused to come out. Only her Peruvian maid, Dolores, is allowed in. She takes Mrs. Ferguson her meals.