It's September 1940, and British intelligence is desperately worried: the Nazi invasion of Britain is imminent. A sinister organisation called The Group, a collection of British and Irish Nazi collaborators, is at work trying to support Hitler's plans. But that is not the only concern for the spy chiefs: the search for double agent 'Archie' - the Soviet spy and British traitor - is proving fruitless, and now they know there's a second traitor, Bertie, also in play. Will they be able to foil the invasion plans without their schemes being leaked to the enemy?