He helped change the course of the largest tank battle in history, then spent the latter half of his life grading papers and translating 17th-century French poetry. This episode unpacks the contradictory life of John Cairncross, the working-class Scot revealed as the fifth man of the Cambridge spy ring.

A linguistic prodigy who topped the civil service exams, Cairncross was an outsider who loathed his elite fellow spies. Recruited by Soviet intelligence in 1937, he passed thousands of documents from Bletchley Park and beyond, yet avoided prison through disciplined interrogation and legal technicalities.

• He scored 100 marks ahead of the second-place candidate on the Foreign Office entrance exam
• He was recruited in Regent's Park by Arnold Deutsch after the ring's elite members failed to win him over
• Working in Hut 3 ...