All of them are friends. One of them is a killer.During the languid days of the Christmas break, a group of thirtysomething friends from Oxford meet to welcome in the New Year together, a tradition they began as students ten years ago. For this vacation, they’ve chosen an idyllic and isolated estate in the Scottish Highlands—the perfect place to get away and unwind by themselves.They arrive on December 30th, just before a historic blizzard seals the lodge off from the outside world.Two days later, on New Year’s Day, one of them is dead.The trip began innocently enough: admiring the stunning if foreboding scenery, champagne in front of a crackling fire,…