The Mystery of the Blue Train

When the luxurious Blue Train arrives at Nice, a guard attempts to wake Ruth Kettering from her slumbers. But she will never wake again - for a heavy blow has killed her, disfiguring her features almost beyond recognition. What is more, her precious rubies are missing.
The prime suspect is Ruth's estranged husband, Derek. Yet Poirot is not convinced, so he stages an eerie re-enactment of the journey, complete with the murderer on board...