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.
High-level secrets are stolen when Calum is mugged outside Thames House. A random robbery or a theft-to-order? Either way there are lethal consequences for the agents and assets of Section D.
MI5 receive a red flash after a radioactive material is detected at an airport. They have intelligence that a dangerous anarchist has secretly entered the country. Dimitri is asked to help find out more by honey-trapping the anarchist's sister, Natalie. The Home Secretary asks Ruth to dinner to talk about a role as his security advisor. The team work out that the leaking of MI5's assets and the foiled assassination on Russian Foreign Minister Gavrik have a connection.
A controversial religious leader parole appeal is refused and he ends up sending two recently released inmates on a revenge suicide mission. One of the inmates at the prison, Ashur, is an MI5 asset who has been passing on useful intelligence from inside the prison. The team want Ashur to find out information and he agrees to help if his daughter becomes a UK citizen. Things don't go to plan when his daughter is kidnapped by the bombers and used as collateral, leaving Harry in an impossible position.
There is new evidence of CIA agent Jim Coaver's involvement in some recent attacks on MI5. Harry ends up kidnapping his former Cold War colleague but soon ends up realising that he's been looking in the wrong place and Jim could be innocent. Harry, with the help of Ruth, sets out to steal Coaver's laptop from under the noses of the Americans at the US Embassy.
Harry finds himself facing deportation to America following the death of CIA agent Jim Coaver. After some new evidence comes to light about the recent attacks on the United Kingdom and the Government's proposed alliance with Russia, Ruth and her former colleagues in Section D free Harry from CIA custody. Harry's suspicions see him going to another of his former Cold War adversaries.