 In the episode , Sir Harry Pearce ( Peter Firth ) is determined to carry out his revenge on London 's FSB head of operations Arkady Kachimov ( Stuart Wilson ) , who in the previous episode purposely withheld intelligence that caused the death of Adam Carter . However , the MI5 team discover he is overseeing a cyber attack in progress , and returned officer Lucas North ( Richard Armitage ) may be involved . In the end , MI5 stop the attack , and Harry kills Kachimov . 
