On the eve of her adored adopted daughter Augusta's twenty-first birthday, and after bullying her harpist Tenella, and telling her great nephew Palin a family secret so dreadful he leaves Tristowell Castle and goes straight down a pothole, old Lady Elizabeth Carn is found securely and determinedly strangled. All sorts of developments in every direction occur, with some thumping subterranean scenes. Inspector Mallet investigates.