Agatha Christie Three Act Tragedy

Hercule Poirot, his amateur detective acquaintance Mr Satterthwaite, and the distinguished Harley Street nerve specialist Sir Bartholomew Strange are amongst the guests at a weekend party being given by the celebrated actor Sir Charles Cartwright at his clifftop house in Cornwall. On the first evening after their arrival there is a cocktail party at which an elderly clergyman, who's only had a sip of dry martini, falls dead. Was it through natural causes or murder