Winter, 1947. Britain's secret services have been penetrated.

The country is more vulnerable than ever - and Soviet leader Joseph Stalin knows it. He decides it is time to send his master of 'Special Tasks' to create extra chaos. But Stalin has a more important motive than mere disruption.

He has a man on the inside who must be protected at all costs - a communist super-spy who has the secrets of the atomic bomb at his fingertips. Freya Bentall, a senior MI5 officer, no longer knows who to trust and is left with one option: to bring in an outsider whose loyalty is beyond question - Cambridge professor Tom Wilde. His task: to find the traitor in MI5.

Bentall has three main suspects and Wilde must get close to them all. That means delving deep into the criminal underworld, attaching himself to the cultural elite of the arts and finding a way into the extreme reaches of British politics. As winter bites and violence erupts, Wilde faces an uphill battle to protect those he loves from merciless killers.

And he knows that one slip will spell disaster for the country - and his family.