Weak. Basically it's Nemesis all over again. The two books may have been written 30 years apart, but when Christie wrote her last Marple book (Nemesis) she seems to have consciously or subconsciously gone back to the Marple story she had in a vault for posthumous publication. The superficial details are different, but the story is exactly the same. 
 The last couple of chapters are quite fast-moving, and for a brief moment I thought she might pull it off. But in the end, it went exactly where you thought it was going. Perhaps if Nemesis didn't exist this book would have earned a higher rating, but as it is I really can't see pushing it from 2 stars to 3. It's just not good. 
 It's said that at the end of her career she liked Marple and disliked Poirot, but she would have done herself and her fans a favor making Curtain rather than this book her final legacy.