Ghosts from the past come back to haunt MI5 chief Harry Pearce, prompting a shocking revelation about his time as a spy during the Cold War.
Harry survives his MI5 employment tribunal and is quickly back on the Grid, where changes have been made to the team: Erin Watts is the new acting head of Section and she has brought with her Calum Reed, a brilliant field officer and technical genius. William Towers, the Home Secretary, is impressed by a report on Ruth that Harry prepared for the tribunal. He suggests to her that she should consider looking for promotion.
Buried secrets are unearthed when Max Witt, a retired spy and Harry's colleague in Berlin during the Cold War, is found murdered by an unknown assassin. Meanwhile Ilya Gavrik, Russia's foreign ambassador and Harry's opposite number in the KGB for much of the 1980s, has flown into London to negotiate a ground-breaking strategic partnership between Russia and UK. Naturally, Harry is deeply suspicious.
Towers insists that Harry put a lifetime of animosity behind him and make friends with the old enemy, a request made even more urgent when the killer of Max Witt now appears to be targeting Gavrik. Gavrik isn't in the UK alone; he has come with his wife Elena, and is joined by his son Sasha, a London-based FSB officer who is tasked with the security of his family while they are in the UK.
Elena was Harry's secret asset in Berlin during the Cold War. Back then, Harry got her to spy on her own husband. But when Elena reveals he has asked her to spy for him again, Harry realises that someone is pretending to be him. Now he must find out who that person is - and why.