Normally, a mere outbreak of petty thefts in a youth hostel wouldn't be enough to interest the great detective Hercule Poirot. However, the warden of the hostel is sister to Poirot's secretary Miss Lemon, and concern for her sister is interfering with Miss Lemon's typing abilities. Poirot finds himself with an intriguing puzzle on his hands, and before long, murder increases the mystery.

Tommy and Tuppence Beresford buy a home and find that they have also acquired a mystery which, with typical enthusiasm, the two set out to solve.