"Triple Cross" was based loosely on the real-life story of Eddie Chapman, believed by the Nazis to be their top spy in Great Britain, although he was actually an MI5 double agent known as "Zigzag". The title of the film comes from Chapman's signature to mark he was freely transmitting by radio, a Morse code XXX. Another meaning of the title "Triple Cross" becomes clear in the final scene of the film. Chapman, sitting at a bar, is asked who he was really working for. In reply, he raises his glass in salute to his reflection in the mirror.