The Campions are snowed in at Christmas, but just when they think it can't get any colder, their holidays take an even chillier turn.

1962, Norfolk. Boxing Day looks set to be a quiet affair for the Campions when they are snowed in at their remote farmhouse, Carterers - until a charabanc full of 'pilgrims' travelling from London to the Shrine of Our Lady in nearby Walsingham crashes into their imposing granite gateposts and the family unexpectedly find themselves playing host to the eccentric passengers.

But any lingering festive cheer is in short supply when a shocking discovery is made the following day, while a terrifying twist reveals that some of the guests are not who they seem. Which - if any - can they trust? Suddenly hostage to events, the Campions are drawn into a fiendish web of espionage as the Cold War comes chillingly close to home.