On the first of December, renowned puzzle setter, loner and Christmas curmudgeon Edie O'Sullivan finds a hand-delivered present on her doorstep. Unwrapping it, she finds a jigsaw box and, inside, six jigsaw pieces. When fitted together, the pieces show part of a crime scene. Included in the parcel is a message: "Four, maybe more, people will be dead by midnight on Christmas Eve, unless you can put all the pieces together and stop me." Edie contacts her nephew, DI Sean Brand-O'Sullivan, and together they work to solve the clues. But when a man is found near death with a jigsaw piece in his hand, Sean fears that Edie might be in danger and shuts her out of the investigation. As the body count rises, Edie knows that only she has the knowledge to put together the killer's murderous puzzle. Only by fitting all the pieces together will Edie be able to stop a killer --- and finally lay her past to rest.

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?

The annual Christmas Game is afoot at Endgame House, the Armitages' grand family home. This year's prize is to die for --- deeds to the house itself --- but Lily Armitage has no intention of returning. She hasn't been back to Endgame since her mother died 21 years ago, and she has no intention of claiming the house that haunts her dreams. Until, that is, she receives a letter from her aunt promising that the game's riddles will give her the keys not only to Endgame, but to its darkest secrets, including the identity of her mother's murderer. Now, Lily must compete with her estranged cousins for the 12 days of Christmas. She will have to keep her wits about her, because not everyone is playing fair, and there's no telling how many will die before the winner is declared.