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 murdered.

Calum is attacked by a gang of street muggers during a routine intelligence-sharing exercise with MI6. The laptop he was carrying is stolen, and contains highly-classified information on MI5's undercover assets. Their names are soon plastered all over the internet, making MI5 look incompetent and putting those top assets' lives in danger.

Two former inmates are released from jail. One is an undercover MI5 asset and the other is an impressionable wannabe suicide bomber who has been radicalised in prison. They are set to launch an attack somewhere in London, but our team must time it right and step in at the eleventh hour to prevent a massive loss of life.