"The Riddle of the Sphinx" is the third episode of the third series of the British dark comedy anthology television programme Inside No.

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.

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.

9 was announced in October 2015, and heavily publicised in January 2016, at which time Alexandra Roach was named as a guest star in the series.

9 episodes "The Harrowing" and "Séance Time", as well as the house of Oscar Lomax in the Shearsmith and Pemberton television series Psychoville.

[10] Close-up shots of the crossword and the blackboard were filmed later, some on the set of "Private View", which introduced potential continuity problems.

[9] BBC executives, according to Shearsmith, saw the episode as a cross between Sleuth and Willy Russell's 1980 play Educating Rita that read as if it had been authored by psychopaths.

[9] The episode stars Roach as Nina, Pemberton as Professor Nigel Squires—whose name is a reference to the crossword setter Roger Squires[9]—and Shearsmith as Dr Jacob Tyler.

A second nina (RIPNHS) was created accidentally and seen at the last minute; by introducing a middle name for Pemberton's character, the writers were able to make it a part of the plot.

[19] In an interview with Connor, published after "The Riddle of the Sphinx" had aired, Pemberton explored the ways in which writing for Inside No.

9 was like writing for a cryptic crossword; in both cases, misdirection is key and utterly arbitrary choices or happenings can serve as important inspirations.

[9] Pemberton's crossword was published in The Guardian on the same day that the episode aired, credited to "Sphinx";[19] for one critic, this explained and justified the answer MYSTERYGUEST.

9 and The Guardian drew inspiration from the collaboration between The Simpsons and The New York Times for "Homer and Lisa Exchange Cross Words" in 2008.

(This was published months before the episode aired, allowing, in Pemberton's words, "some very clever people" to guess that he was appearing on the programme before it was officially announced.)

Squires places Nina—for whom paralysis is setting in—on a chair, and goes back to the crossword, filling out UNDERSLIP; asking about the underwear young women wear, he slides his hand up Nina's skirt and kisses her on the mouth.

Tyler tells Squires that he cannot call the police, as the crossword displays premeditation: the KNOWITALL received a MYSTERYGUEST at number NEUN, resulting in ASPHYXIATION.

Tyler reveals PUFFERFISH, predicting, with reference to crossword answers, that a DOWN AND OUT will find Charlotte wrapped in her UNDERSLIP in SWAMPLANDS, incriminating Squires.

[18][25][26] William Shakespeare's Titus Andronicus, the 1973 horror film Theatre of Blood, and the work of Anton Chekhov were also identified as possible influences.

[18] Mark Butler, writing for inews.co.uk, noted the references to Greek tragedy, arguing that the episode "followed a long tradition of grotesque dramatic reveals".

[26][27] On the episode's audio commentary, Shearsmith denied that the nina RIPNHS should be read as a political statement, namely "Rest In Peace National Health Service".

[9] Similarly, in his interview with Connor, Pemberton noted that "most people" think that the nina is political, but that it was an accidental introduction, and only woven into the plot once the crossword had been completed.

[28][29][30] According to Pemberton, "Because each episode is so wildly different there was nothing really linking them other than the fact they were all inside a Number Nine, I just thought it would be nice to have an object that you could hide and just have there on every set.

[26] At the end of the fourth series, Butler listed "The Riddle of the Sphinx" as the second strongest episode of Inside No.

[36] Mellor, similarly, praised the intelligence of the plot, provided one takes "on faith the unlikely notion that a mother and her new husband would have no contact with her children from a previous marriage, not even recognising them as adults".

[18] "From the lightning flashes that punctuate hints and story shifts to the wordplay and in-jokes peppered through the script", Mellor said, the episode is highly precise.

Nina is shown drinking from the poisoned cup one of two times she admiringly calls Squires "devious" (a hint at her true feelings about him) and once again when she emphasises the word "plan".

[18]The actors' performances were also commended,[36] with particular praise for Roach,[18][36] who was characterised as "funny, likeable and endearingly crude as Nina, then captivating and clever when the charade drops".

[18] Patrick Mulkern, writing for RadioTimes.com, also praised the "zingy funny lines" in the earlier part of the episode,[25] and Butler commended the "smart, gentle humour" offered by the contrast between Nina and Squires.