The Adventure of the Sussex Vampire

Mr. Robert Ferguson thinks his wife might be a vampire. He's caught her sucking their baby's neck and since that discovery, she has locked herself in her room, only allowing their Peruvian maid to bring her food. Her behaviour had been strange before that and she had struck Ferguson's son from an earlier marriage twice. The boy, fifteen, has been left handicapped by a childhood accident. Holmes is immediately convinced he has solved the case upon hearing the story but he follows Ferguson back home to clear the mystery.
"The Adventure of the Sussex Vampire" is part of "The Case-Book of Sherlock Holmes".