Sandra Sim dies a squalid death, murdered behind a wooden hut, on a quayside, on a wet November night. What worries Detective Superintendent, Bill Smellie, most is the method of her death: she is the second prostitute in a month to be strangled with a blue rope, a pound coin placed under her tongue, and her panties stolen. During a massive investigation, the police receive a letter from the strangler, threatening more murders. Then a third woman is found dead -- but this time the killer has made a mistake...