When Joan Thursday asks Morse how he's getting on living with Detective Sergeant Strange, he replies "It's not the Yellow House". This is a reference to the fact that the painters Vincent Van Gogh and Paul Gauguin lived for a short time together in the Yellow House in Arles, France. Their relationship was a stormy one and they often quarreled.
Millie Bagshot tells Morse that "since '51", the attention of the security forces has been focused on Cambridge, rather than Oxford, University. It was in 1951 that Guy Burgess and Donald Maclean, two top-ranking Foreign Office officials, successfully defected to Russia before they could be arrested as Russian agents, which both had been throughout their adult lives. They had first been recruited by the Russians whilst undergraduates at Cambridge in the 1930s.
When Millie Bagshot tells Morse that Communist spies may have infiltrated HMG (that is, Her Majesty's Government) at Cabinet level "or even higher", she is referring to a long-whispered rumor that Harold Wilson, the Prime Minister in the year this story is set, was a Soviet agent. This view was seriously held by James Jesus Angleton, a senior figure in America's Central Intelligence Agency, and certain Tories in Britain were, for their own ends, only too happy to spread the whisper, despite Angleton's offering no proof for his notion and his being, rather obviously, a paranoid right-wing fanatic. No evidence was found by British security services, but the rumor surfaced again in 1976 when Wilson abruptly resigned from public life at the age of 60. The real reason for his resignation emerged only after his death in the 1990s; he was suffering from Alzheimer's disease.
Ellie Haddington's performance seems designed to bring back memories of her earlier role as spy-mistress Hilda Pierce in the Foyle's War (2002) series, whilst Richard Durden is made up and costumed to resemble the real-life spy and traitor Anthony Blunt.
There are various references to BBC sitcoms of the 1960s concealed within this episode. Pfuscher's house is said to be in "Sebastopol Terrace", which was the address for Eric Sykes and Hattie Jacques in their long-running BBC series Sykes (1972). Joe Dozier refers to his wife contemptuously as a "silly moo", the derisive term Warren Mitchell so often flung at Dandy Nichols in Till Death Us Do Part (1965). On the wall outside the Doziers' shop is an advertisement for "Grimsby Pilchards", the non-existent product endorsed by Diana Day in the earlier episode "Trove" and originally featured in the famous segment of "Hancock" entitled "The Bowmans". A billboard showing Diana Day endorsing this product is seen in several "Endeavour" episodes.