This is early enough in the Poirot series that Christie isn't annoyed with him yet so his idiosyncracies are quite mild and this allows the story to shine through much more. 

I read this years ago and thought I didn't remember the plot. However, as I went it must have triggered my memory because I figured out the solution about halfway through. It didn't diminish from my enjoyment of a well-executed novel from the queen of mysteries.