What is the main conflict in Peril at End House, by Agatha Christie by Agatha Christie?

In Peril at End House, retired detective Hercule Poirot becomes involved in the seeming murder attempts against Nick Buckley, the attractive young owner of a family mansion called End House. Nick's cousin Maggie Buckley is killed, presumably mistaken for Nick, and Poirot gets Nick to reveal that she's secretly engaged to airman Michael Seton and heir to a large fortune. After Nick becomes poisoned by chocolates laced with cocaine, Poirot eventually realizes that he's been mistaken all along. Nick was never the intended victim. She murdered Maggie to take her place as Seton's supposed fiancée and heir.