Hercule Poirot doesn't need all his detective skills to realise something is troubling his secretary, Miss Lemon - she has made three mistakes in a simple letter. It seems an outbreak of kleptomania at the student hostel in which her sister works is distracting his usually efficient assistant. Deciding that desperate times call for desperate measures, the great detective agrees to investigate. Unknown to Poirot, however, desperation is a motive he shares with a killer.