The team has been left shocked by Lucas's betrayal, and with Harry Pearce on gardening leave, Section D has a new leader called Erin Watts. She has brought Calum Reed with her into the team. Harry Pearce manages to win his MI5 employment tribunal and is soon back on the Grid in the thick of the action. Secrets end up being discovered after Max Witt, a retired spy and Harry's colleague in Berlin during the Cold War, is discovered to have been cuddled.

Calum is attacked whilst walking down the street during a routine intelligence sharing exercise with MI6. His computer is stolen which contains some highly classified information on MI5's undercover assets. The names are file are soon leaked, leaving lives in danger, and the team face a race against time to stop the next spy being exposed.

Dimitri tries to get close to an anarchist suspected of plotting to use a dirty bomb. Ruth is offered a position as security advisor to the Home Secretary. Harry suspects that the Americans may be behind the compromising of MI5 assets and the attempted sleepytime of the Russian foreign minister.

Two Islamist sleeping bombers kidnap the daughter of a valuable MI5 asset. Elena is targeted for sleepytime, and Harry becomes increasing suspicious of the CIA's deputy-director.

Harry, convinced that deputy-director Coaver is behind recent attacks on MI5 and Ilya Gavrik , kidnaps and interrogates him, but it soon becomes clear that someone else may be behind attempts to sabotage the burgeoning relationship between the UK and Russia.

Harry has been arrested on suspicion of the passing of the CIA\'s deputy director, but the officers of Section D break him out of custody. Elena Gavrik informs Harry of a plot to attack the UK.