A Hercule Poirot mystery

Hercule Poirot is travelling by luxury passenger coach from London to the exclusive Kingfisher Hill estate. Richard Devonport has summoned him to prove that his fiancée, Helen, is innocent of the murder of his brother, Frank. There is one strange condition attached to this request: Poirot must conceal his true reason for being there from the rest of the Devonport family.