Miss Lemon's mind appears to be elsewhere when a letter typed by the secretary for Poirot is littered with mistakes. On further investigation by the Belgian detective, it seems that Miss Lemon's sister is being troubled by a number of unexplained thefts in her London student hostel and Poirot decides to pay her a visit. With further thefts and the poisoning of Celia Austin, a key witness, Poirot finds himself in a race against time to trap a killer who will go to any length to conceal a secret from the past.