I very much enjoyed this Christie novel the first time through, but on rereading in publication order, it wore a little thin. Bobby Jones & Frances Derwent are too much like pale imitations of Tommy & Tuppence in The Secret Adversary and Bundle Brent & Bill Eversleigh in The Seven Dials Mystery. The solution to the titular question is quite clever -- but it was so clever that I remembered it, further dampening my enjoyment in rereading. There also wasn't the same insight into character that Christie shows in other novels, particularly Poirot and Miss Marple novels. So all in all enjoyable, but not one of her greatest.