"The Riddle of the Sphinx" is the third episode of the third series of the British dark comedy anthology television programme Inside No. 9. It first aired, on BBC Two, on 28 February 2017. The episode was written by the programme's creators, Steve Pemberton and Reece Shearsmith, and directed by Guillem Morales. "The Riddle of the Sphinx", which is set in Cambridge, stars Alexandra Roach as Nina, a young woman seeking answers to the Varsity cryptic crossword, Pemberton as Professor Nigel Squires, who pseudonymously sets the crossword using the name Sphinx, and Shearsmith as Dr Jacob Tyler, another Cambridge academic. The story begins with Nina surreptitiously entering Squires's rooms on a stormy night and being discovered; this leads to Squires teaching her how to decipher clues in cryptic crosswords. The plot of "The Riddle of the Sphinx" revolves around the clues and answers to a particular crossword puzzle. The idea to focus an episode on crosswords came from Pemberton; he had long been a fan of cryptic crosswords, but particular inspiration came from Two Girls, One on Each Knee: The Puzzling, Playful World of the Crossword, a non-fiction book by Alan Connor. The crossword featured in "The Riddle of the Sphinx" was set by Pemberton, and was published in The Guardian on the day the episode aired, credited to "Sphinx". This crossword contains multiple ninas-hidden messages or words. Along with many of the crossword's answers, one nina is integral to the episode's plot. A second was introduced accidentally, and then incorporated into the episode. A third nina is an Inside No. 9 in-joke. Influences for "The Riddle of the Sphinx", which emphasises gothicism over comedy, include Anthony Shaffer's 1970 play Sleuth, the work of Anton Chekhov, and the 1989 film The Cook, the Thief, His Wife & Her Lover. Critics responded extremely positively to the episode, lauding its writing and the precise attention to detail in the production. Commentators noted that the episode was very dark, and probably the cleverest episode of Inside No. 9 to date. Roach's performance was praised, as was the direction of Morales.