Section D are faced with their toughest task yet as they are forced to track one of their own. Following Lucas’s shocking revelations about his former life and double identity, Harry instructs the team to find him at all costs. But as one of MI5’s most skilled agents, Lucas is not an easy man to find.

Harry enlists the help of an ex-internal affairs specialist, Alex White, who promises he will be able to deliver Lucas, while the rest of the team struggle to come to terms with his betrayal.

Meanwhile, out on the run, Lucas is letting nothing stand in his way as he gets increasingly desperate to track down the Albany file and flee the country with the love of his life, Maya.

Elsewhere, Harry is put in a difficult position when he discovers Ruth’s life is in danger. He is forced to choose between his loyalty to MI5 and his heart. But which will it be? The decision reaches its climax in an emotional showdown when Harry comes face to face with Lucas with shocking consequences.