Hercule Poirot is a fictional character, and not even my favorite. I've read Curtain at least once before, although long enough ago that I only remembered a small subplot, and yet I teared up a bit when Hercule Poirot died.
Agatha Christie got tired of Hercule Poirot long before her readers did, but there was little she could do other than have Ariadne Oliver complain about her own foreign-born detective. It must have been cathartic to kill him off, even if she knew no one would read of his death until his creator was dead or dying herself. Written during WWII, when Christie (along with much of London) spent sleepless nights hiding from the Blitz in the Underground, Curtain finally saw publication in 1975, shortly before Christie's death. Christie avoided any "dating" references, but the manuscript's age shows. There are no TVs at Styles (reborn as B&B of sorts), and Styles itself is still semi-rural despite being close to London and its sprawl. Cash-poor gentry still live on declining shares rather than entering professions, and a woman with a university degree is called a doctor's "secretary" instead of his research assistant. As a teenager, I didn't notice quite how dated Curtain is - 1975, although barely a decade in the past was the semi-distant past of my childhood - but this time, the differences between timeless-1945 and real-1975 jumped out at me.
My copy of Curtain says that it's "Hercule Poirot's Last and Greatest Case." I found it to be more of a middling Christie, with a classic set-up. Poirot, now wheelchair bound, invites the recently widowed Captain Hastings to join him at Styles to help him catch one last murder. "X" has killed - not directly, but insidiously, through other people - at least five times, and Poirot believes X both will kill again and is currently staying at Styles. Hastings job is to stop X. Needless to say, Hastings not only does not identify X but also becomes unwittingly enmeshed with X's plot. It's up to Poirot to solve the mystery and serve justice of a sort. As in Murder on the Orient Express, the solution may be more "just" than "right," only this time, the instrument of justice also dies before the final page.