Three Act Tragedy
Agatha Christie
 When a clergyman dies at a dinner party thrown by stage actor Sir Charles Cartwright, it is thought by nearly everyone (Poirot included) to be an accidental death. Shortly afterwards, however, a second death in suspiciously similar circumstances and with many of the same people present puts both Poirot and a team of sleuths on the trail of a poisoner whose motive is not clear. The solution to this mystery is one of Christie's classic pieces of misdirection and relies on a plot device which has been widely imitated. Poirot reveals that the first murder - in which the murderer could not have predicted who would get the poisoned glass and had no motive to kill the eventual victim - had only been a "dress rehearsal" for the second murder.