"The Riddle of the Sphinx" is the third episode of the third series of the British dark comedy 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.

Plot

On a stormy night, Nina lets herself in to a University of Cambridge room, where she is found by Professor Nigel Squires. He is holding a gun, but it is not loaded. Nina's boyfriend Simon is a fan of cryptic crosswords, she explains, but she is never able to help him. She has come to the rooms of Squires—a classicist who sets crosswords for Varsity as the Sphinx—to seek the answers to the next day's crossword. Squires sets about teaching Nina: "I teach wild creature without hospital building" results in ARCHITECTURE, which Simon studies. They turn to the clues for the next day, beginning to fill a large grid. Squires uses the name of the Sphinx because she would asphyxiate and consume those who failed to answer her riddle: she was, he says, "devious and deadly". Squires makes tea, as Nina looks at his trophies. A picture of Squires with his late wife draws her attention, and they discuss the cut-throat world of competitive crosswording. Nina has answered DOWNANDOUT and WRAP; Squires answers DESI and helps with TRENT. Squires asks about Simon, but catches Nina in a lie; her excuse is that she only wants to learn. Together, they deduce SWAMPLANDS, meaning "bog". Nina, though, suggests that it should be bogs, otherwise Squires would be cheating. Suddenly spluttering, Squires takes a seat, as Nina begins on the next clue. She now displays clear proficiency, answering ASPHYXIATION.

Squires drops his cup, as Nina continues to fill in the crossword, including SOWERBERRY and KNOWITALL. Squires is apparently paralysed in his chair, watching. Nina is a marine biologist, and has acquired tetrodotoxin from a pufferfish, which causes paralysis and asphyxiation. Simon was actually Nina's brother, and is visible on the photo of Squires and his wife. Simon had reached a crosswording final only to be beaten by Squires after the latter challenged that a u looked more like a v. Depressed by the defeat, Simon had killed himself. Squires, to Nina's shock, is unharmed. He leaps up to fill out NEUN and ASPS, revealing the nina ISWAPPEDCUPS within the crossword. As Nina induces vomiting, Squires makes a phone call. He had been warned by Dr Jacob Tyler, an old friend and Nina's supervisor. 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.

Nina is left alone until Tyler enters. He tells Charlotte—"Nina"—to hang on, before turning to the crossword. With Squires, he works out MYSTERYGUEST. Tyler reveals that he has no antidote for Charlotte, and will not call the emergency services. Instead, he wants Squires to eat Charlotte, as the Sphinx would. 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. This is, Tyler says, his revenge.

Tyler cuts from Charlotte's buttocks, frying a strip of her flesh on a stove. Squires tells of how he began an affair with Monica, Tyler's wife and mother of his twin children, destroying Tyler's career. Tyler hands Squires the flesh; he eats, fearing Charlotte will die. Tyler reveals that he hates cryptic crosswords, and how his son entered the Cambridge Crossword Competition, attempting to beat his mother's new husband: Squires. Squires realises that Charlotte is Tyler's daughter. Charlotte and Tyler sought revenge on Squires, but Tyler changed plans so Squires would include clues in the crossword. However, Simon's autopsy—Tyler explains—revealed that Simon and Charlotte were actually Squires's children, meaning Monica and Squires's relationship began earlier than he previously thought. Charlotte is past saving, and Tyler places a bullet on Squires's desk, reminding Squires of the principle of Chekhov's gun. Squires confirms that his middle name is Hector as he weeps over Charlotte, and Tyler circles something on the crossword. Charlotte is dead, and Squires loads the gun, placing it in his mouth. Blood splatters over the crossword and a second nina: RIPNHS.

Production

The third series of Inside No. 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. The series began with the Christmas special "The Devil of Christmas" (December 2016) and continued with "The Bill" (February 2017), the latter of which was the first of a run of five episodes, of which "The Riddle of the Sphinx" was the second. The episode was first aired on 28 February 2017. It was shown on BBC Two at 10:00pm, clashing with first episode of the third series of Catastrophe, the acclaimed Channel 4 comedy.

"The Riddle of the Sphinx" was written in the summer of 2015, one of the last of the series, and was filmed in December that year. Most of the episode was filmed in Langleybury, a country house in Abbots Langley, Hertfordshire, which had previously been used for the Inside No. 9 episodes "The Harrowing" and "Séance Time", as well as the house of Oscar Lomax in the Shearsmith and Pemberton television series Psychoville. The opening exterior shot was filmed in a court of Gonville and Caius College, Cambridge. The idea to set the episode during a storm, which gave it a gothic quality, was Morales's. For Shearsmith and Pemberton, this was an example of an advantage to having a third party direct episodes, in contrast to "Cold Comfort" and "Nana's Party" from the second series, which they had directed themselves. Close-up shots of the crossword and the blackboard were filmed later, some on the set of "Private View", which introduced potential continuity problems. The episode was filmed after two days' rehearsals. The script was 32 pages in length, resulting in a first cut that was 38 minutes long. The BBC permitted the final version to be a little longer than the half hour typical of Inside No. 9, but the production team still needed to lose several minutes in the edit. The final run-time for the episode is 31 minutes and 35 seconds.

"The Riddle of the Sphinx" was characterised by Pemberton as a thriller in the style of Anthony Shaffer's 1970 play Sleuth, while Shearsmith identified the episode as particularly theatrical, in the style of Sleuth and Ira Levin's 1978 play Deathtrap. 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. The executive producer Jon Plowman suggested that the first half of the episode plays out like a radio play. The word-play and tea-drinking in this part of the episode are, in the writers' opinion, very English; the episode then changes character. Civilisation is "stripped away", resulting in the episode having the elements of a Greek tragedy. A further inspiration was the 1989 film The Cook, the Thief, His Wife & Her Lover; at the end of the episode, Squires faces a situation that mirrors one faced by Michael Gambon's Albert Spica.

An initial idea for the plot saw only two characters, but a third, for the writers, introduced a compelling dynamic; keeping the cast small also served to keep production costs relatively low. The episode stars Roach as Nina, Pemberton as Professor Nigel Squires—whose name is a reference to the crossword setter Roger Squires—and Shearsmith as Dr Jacob Tyler. Roach and Shearsmith had previously collaborated on the television programme Hunderby. Roach said in an interview before the episode's airing that she is drawn to darker scripts; she said that she loved Inside No. 9, describing the episodes as "very gruesome, psychological thrillers, [which] always have great twists". She described Nina as highly naive. Two doubles were used in the episode in place of Roach. One was used in the exterior shot at the start of the episode; a second was a "bottom double" seen towards the episode's close.

Shearsmith identified "The Riddle of the Sphinx" as one of his two favourite episodes of the series, along with "The Devil of Christmas". For the writers, the episode was not comedic, and they expressed awareness of the differences between episodes of Inside No. 9 like "The Riddle of the Sphinx" and the expectations of viewers for television comedies. The pair acknowledged the particular complexity of the plot, with Pemberton saying "If you missed two sentences in a row, you'd be like 'what?' Even I was thinking how are people following this? It's insane."

Crossword setting

Pemberton had long been a fan of cryptic crosswords, and he was inspired to develop the episode by reading Two Girls, One on Each Knee: The Puzzling, Playful World of the Crossword, a non-fiction book by Alan Connor. Pemberton had first met Connor through an appearance on the BBC game show Only Connect, on which the latter was working as a question setter. Subsequently, he received a copy of Connor's book. Reading Two Girls, One on Each Knee led Pemberton to ask whether one could "dramatize doing a crossword, which is so un-dramatic?" Pemberton explained that he and Shearsmith relish the challenge set by writing limitations, such as basing an episode around a crossword. Such constraints, he felt, encourage them to produce their best work.