Sarah Caulfield was the CIA Liaison Officer to Section D of MI5 in Series 8. She first appears in the first episode of the series in which Harry has been kidnapped by a former Indian Intelligence Officer searching for a supply of uranium which would have been used to legitimise the Iraq War. She replaces Libby McCall, who's found out to be part of this uranium plot.
During her time in the series, she started up a romantic relationship with MI5 Officer Lucas North but this relationship didn't last as she was forced to go on the run when Lucas found out that she was part of an international organisation called 'Nightingale' whose goal was to spark a war between India and Pakistan, in the thought that it would prevent the Taliban from gaining access to nuclear weapons. She also had a quite bitter rivalry with the MI5 intelligence officer Ros Myers.