The Smiler With The Knife By Nicholas Blake

Detective Nigel Strangeways, and his explorer wife Georgia have taken a cottage in the countryside. They are slowly beginning to adjust to a more relaxed way of life when Georgia finds a mysterious locket in their garden and unwittingly sets the couple on a collision course with a power-hungry movement aimed at overthrowing the government. It will take all of Nigel's brilliance and Georgia's bravery if they are to infiltrate the order and unmask the conspirators.