All aboard, but attention! Sleeping passengers on this train may never wake up. In the early hours of Christmas Eve, the overnight train from London to the Highlands is derailed, along with its passengers' festive plans. With the train stuck in the snow in the middle of nowhere, the passengers only have each other, and not everyone will make it to their Christmas celebrations. As a killer attempts to take out the passengers one by one, former detective Roz Parker can't resist one last investigation, but the locked-room murder is a formidable puzzle for even the most seasoned investigator.