Agatha Christie 
The Affair at the Victory Ball [EPUB ebook] 

In Agatha Christie’s ‘The Affair at the Victory Ball, ‘ Poirot is enlisted by Chief Inspector Japp to assist in the investigation of a murder at a costumed Ball. Six attendees form a circle of suspicion when a young aristocrat and his fiancee are found dead. Poirot then makes an interesting discovery about the costumes worn by the six friends. This short story originally appeared in the March 7, 1923 issue of The Sketch magazine.