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 wife has been sucking their baby son's blood. The woman is Mr. Ferguson's second wife. By his first wife, he has an adolescent son of 15, who suffered an unfortunate accident as a child and now does not have the full use of his legs, although he can walk. His name is Jack, and he 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.