Penrose is an eccentric old man in possession of some dazzling gems, which he won't insure. When Dr Thorndyke is alerted to a burglary at his house, a scrap of paper is found with the word 'lobster' on it along with two Latin words. Meanwhile, Penrose has fled in panic after a car accident. The police are clearly mystified, but Thorndyke in his indelible style is on track; hunting down a fugitive, testing a theory, and getting to the bottom of a tantalising, complex mystery.