A shriek rang sharply out only to be chopped in two-exactly as though a hand had been clamped roughly on the screamer's mouth. It was followed by brusque, striding footsteps and the soft closing of a door. It was purely by chance that David Beddoes and Alison Young stumbled on the strange house in Hampstead-a thick fog made it easy to mistake their destination-and thus set in motion the extraordinary train of events that was to end so sensationally...