The countdown to invasion begins...

It's September 1940, and British intelligence is on high the Nazi invasion of Britain is imminent. The German navy is assembling a vast fleet in Rotterdam to ferry men and materiel across the Channel.

Meanwhile, a sinister organisation called The Group a collection of British and Irish Nazi collaborators is at work within the UK trying to pave the way for the Wehrmacht.

The Invasion Warning Sub-Committee is established to counter this threat, and Charles Cooper, coming in from the cold after the disintegration of The Annexe, joins up and is tasked with cracking this nefarious network.

But that is not the only concern for the spy the search for double agent 'Archie' the Soviet spy and British traitor is hotting they are now confident he works within MI6, and an elaborate scheme is set up to narrow down the list of suspects To make things worse, they now know for certain that a second traitor, 'Bertie', is also in play.

With suspicion at paranoia pitch, can MI6 clean house before the Nazis arrive to help them do it?