Poirot is enlisted by Japp to help solve a mystery that took place on Bonfire Night in a mews flat. A Mrs. Allen was found shot, apparently a suicide, but she was holding the gun that killed her in the wrong hand, and foul play is suspected. Furthermore, the ash-tray in the room contained the stubs of Turkish cigarettes smoked by one Major Eustace, a disreputable acquaintance.The victim was engaged to be married and seemingly had no cause to take her own life. Did she? Or was it murder in the mews?