Hickory Dickory Dock (Hercule Poirot, #31)

An outbreak of kleptomania at a student hostel was not normally the sort of crime that aroused Hercule Poirot’s interest.

But when he saw the list of stolen and vandalized items – including a stethoscope, some old flannel trousers, a box of chocolates, a slashed rucksack and a diamond ring found in a bowl of soup – he congratulated the warden, Mrs. Hubbard, on a "unique and beautiful problem". The list made absolutely no sense at all. But, reasoned Poirot, if this was merely a petty thief at work, why was everyone at the hostel so frightened?

PS: did you know that Mrs. Hubbard is Miss Lemon's sister? That's how the case came to M. Poirot in the first place!