Old, ailing, and confined to a wheelchair, Hercule Poirot returns to Styles, the country house where he solved his very first English case. The grand estate is now a shabby guesthouse run by a bickering elderly couple, and Poirot summons his old friend Captain Arthur Hastings to join him there. Hastings, recently widowed, finds his companion physically diminished but mentally as sharp as ever, and learns the reason for the urgent invitation: Poirot is certain that a murderer is already among the guests. The detective has studied five seemingly unconnected killings from the past and concluded that a single hidden figure, whom he calls only 'X', was present at each. Now X is at Styles, and Poirot believes another death is being engineered. Yet this killer leaves no fingerprints and commits no act that the law could touch, working instead through suggestion and the weaknesses of others. Among the houseguests is Hastings's own grown daughter, Judith, drawing him into the danger in ways he never anticipated. As tensions rise within the close circle of residents, Hastings becomes Poirot's eyes and legs, observing the suspects while struggling to grasp the elusive pattern his friend perceives. Set against the faded familiarity of the house where it all began, this final chapter brings the two companions back together for one last, gravely personal investigation.