 In the fourth episode of the eighth series , Oleg Darshavin , who had been one of Lucas 's interrogators in Russia , approaches him for help in stopping a planned bombing by Sudanese extremists ; Lucas takes the case despite Harry 's suspicion that he developed Stockholm syndrome during his captivity . Harry 's suspicions are solidified when Lucas goes off @-@ piste , but ultimately , Lucas is able to bring Darshavin in and takes him into FSB custody after uncovering his involvement in the plot , while Ros stops the bomb . During the same series , Lucas conducts an affair with CIA liaison Sarah Caufield ( Genevieve O 'Reilly ) , who is later revealed to be an operative of " Nightingale " , a multinational shadow organisation bent on a New World Order . When he discovers this and confronts Sarah about her involvement , she flees . She returns in the series eight finale , and before she is assassinated , she reveals that Nightingale is attempting to provoke a nuclear war between India and Pakistan . In the end , Lucas and Ros go to a hotel rigged to explode , set up by Nightingale , to save Pakistan President Mudasser and Home Secretary Andrew Lawrence before the building explodes ; Lucas rescues Mudasser , who calls off the impending war , but Ros is killed in the explosion before she can get Lawrence to safety . 
