Shadowy Whitehall intellegence figure Stephen Fisher arrives to tell the team about their coldest case so far: a 160-year-old murder. Then Jack stuns trhe others by announding that he's quitting.

Did a young diplomat commit suicide, or was she murdered? She had suffered personal tragedy and professional disgrace shortly before her death, but Sandra isn't convinced. Ignoring warnings from her boss, Sandra explores a possible cover-up at the Foreign Office