What is the main conflict in Curtain by Agatha Christie?

he story follows the main character, Arthur Hastings, where he embarks on a trip to stay in the Styles guesthouse, which is a house that he once lived in when his friends owned it. Hastings meets his good friend, and famous investigator, Hercule Poirot, who is also staying the Styles guesthouse. Hercule Poirot is a French private investigator that is world renown for solving some of the most perplexing mysteries. When Hastings arrives at the guesthouse, Poirot shows Hastings a paper that describes five different murder cases. Poirot tells Hastings that the murderer, which he names X, is responsible for all five cases.