The unthinkable has happened at London's Peculiar Crimes Unit. In the depths of winter, a member of staff has been found murdered, and everyone who works there is suddenly a suspect. But Arthur Bryant and John May, the eccentric elderly detectives who run London's strangest crime division, aren't on hand to solve the crime. They've ventured into the heart of the English countryside, only to become stranded on a desolate snowbound section of road. As the blizzard worsens, Bryant and May attempt to solve the crime long distance using only their mobile phones. Unfortunately, their situation is about to get very much worse...