 In the seventh series , Harry Pearce ( Peter Firth ) is able to secure Lucas ' release from prison in exchange for a Russian spy imprisoned in the UK . However , Arkady Kachimov ( Stuart Wilson ) coerces Lucas to work against MI5 , to assist Russia in launching a cyber attack against the UK . In the first episode , Lucas aids section chief Adam Carter ( Rupert Penry @-@ Jones ) in rescuing a kidnapped soldier and preventing the bombing of a Remembrance Day memorial service ; the rescue results in Adam 's death . In the second episode , the team begins to suspect Lucas of working for the Russians , but he later reveals that he is actually trying to stop Arkady and the attack ; the team succeeds in stopping the attack and apprehends Arkady . Following Adam 's death , Lucas is reassigned as Senior Case Officer , and handles Adam 's assets . In the third episode , Lucas recalls being tortured and asked for details of " Sugar Horse " , a top secret MI5 operation involving the recruitment of Russians who move into top positions in the Russian government . In the penultimate episode , Harry orders Lucas to Moscow to uncover the identity of a Federal Security Service ( FSB ) mole in MI5 who is sabotaging Sugar Horse and framing Harry . There , Lucas discovers that Connie James ( Gemma Jones ) is the mole . In the season finale , Section D learns that operation " Tiresias " , the Russian equivalent of Sugar Horse , is going to detonate a nuclear bomb in London . Lucas and Ros Myers ( Hermione Norris ) save Connie from the authorities in order to enlist her help in stopping the attack . After finding the bomb , Connie disarms it , Lucas asks her who sold him out to the Russians and she admits it was her with her final words but she dies in the process as the bomb goes off seconds later . 
