If you read many works of Christie's, you'll find a pattern. She doesn't let you guess who the culprit is, she just won't- many times she hides it successfully, but this time it's pretty obvious. At some point of the book you will know who it is, but to get rid of that idea on your head, she puts some 'plot twist' that isn't even surprising. Also it is her style for the book to be all about looking for the culprit, not about the characters' struggles to survive. Honestly this book is pretty much a let-down compared to her other works, but considering the fact that this was published 80+ years ago, Agatha Christie is still, undoubtedly, a genius.