When a series of apparently minor thefts plagues a university hostelry run by Miss Lemon's sister, Poirot is recruited to investigate. Celia Austin, a pharmacological major, confesses that she is a kleptomaniac and responsible for most of the thefts but denies stealing several objects including a stethoscope, light bulbs, and a student rucksack. Furthermore, she claims to know the other thief and vows to help return the missing items. Unfortunately someone substitutes an overdose of morphine for one of her sleeping powders, and she takes the identity of the thief to the grave. Japp connects the murderer's m.o. with a cold case he had investigated ten years earlier, but the prime suspect in that crime, a powerful statesman, now lies dying in a local hospital. Despite numerous obstacles, Poirot is able to link the killing to an international diamond smuggling ring but not before the murderer claims more victims.