This is one of my favourite memories of a Christmas read. I first read this when I travelling to my mum's for Christmas a few years ago and was stuck at an airport because of a snow storm.
The story unravels in typical Christie fashion... A family gathering at a country house, a bunch of secrets, two strangers, a butler, and the inevitable murder - witnessed by all as a scream so loud that the whole house is shaken and tries to break down old Mr Lee's door. It is locked from the inside.
What ensues is the usual investigation and the unveiling of the different characters of the Lee family. Secrets of Mr Lee's past are revealed bit by bit and there is a lot of marveling about how the murder could have been committed in a locked room with no means of exit and without being seen by the house party.
As far as Christie stories go, this one is slightly more gory. Apparently, Christie's nephew had complained to her that her stories did not contain enough bloody details.
Still, it does follow the formula of most of her books and spoils the reader with red herrings and possible leads until the very end. This one is a great story to guess along as all the clues are there - no last minute revelations.