 The eighth series sees the return of Oleg Darshavin , one of Lucas 's former interrogators during his imprisonment . During his interrogations , Lucas had developed Stockholm syndrome , and his interrogator had managed to deconstruct him almost to the point of being turned and wanting to die . Lucas also becomes involved with CIA liaison officer Sarah Caufield . The initial power struggle between them ends up in a " torrid love affair , " which is monitored by the team . When Ruth Evershed ( Nicola Walker ) returns in the series , Lucas " really takes control of her position " , in a similar manner to Harry and Ros 's actions during Lucas 's return . Armitage described Lucas 's friendship with Ros as a " good combo " ; while Lucas likes to play the " maverick " with " all guns blazing " Ros would " slip around the back and silently slit a throat . " 
