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 and two Latin words, and 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.