All aboard, but beware! Passengers who sleep on this train may never wake up.

In the early hours of Christmas Eve, the sleeper train from London to the Highlands derails, along with the festive plans of its travelers. With the train buried in snow in the middle of nowhere, the passengers have only each other, and not all of them will reach their holiday celebrations.

As a killer tries to pick passengers off one by one, former Met Detective Roz Parker can't resist one last investigation, but murder in a locked room is a formidable puzzle for even the most seasoned investigator. As accusations begin to fly, the group of travelers fractures and unexpected alliances form. Can Roz find the culprit before anyone else is lost?