Section D face their toughest challenge yet when they are tasked with tracking one of their own, as the spy drama concludes. Following Lucas North's shocking revelations about his former life and double identity, Harry Pearce instructs the team to find him – at any cost.As one of MI5's most skilled spies, Lucas is not an easy man to find. Harry enlists the help of ex-internal affairs specialist Alec White who promises to locate the fugitive, while the rest of the team struggle to understand why he would betray them.Out on the run, Lucas is getting increasingly desperate to track down the Albany file and also to flee the country with the love of his life, Maya Lahan. It seems that he doesn't care who or what gets in his way.Meanwhile, Harry discovers Ruth Evershed has been put in grave danger and he is forced to choose between his loyalty to MI5 and his heart. This decision culminates in an emotional showdown when he finally comes face to face with Lucas, with shocking consequences.Lucas North is played by Richard Armitage, Harry Pearce by Peter Firth, Alec White by Vincent Regan, Maya Lahan by Laila Rouass and Ruth Evershed by Nicola Walker.