‘I was just showing off,’ admits Steve Pemberton’s character in ‘The Riddle Of The Sphinx’.

The line may be a sly acknowledgment of he and co-creator Reece Shearsmith’s habit of often delivering a riddle wrapped up in a mystery, inside an enigma.

This latest episode of Inside No 9 will be possibly one of the most divisive instalments of the anthology series. Not that there’s anything, particularly, extremely leftfield about the story, and – at the other end of the spectrum – neither will any audience member be able to coherently argue that this is the sort of thing we’ve seen many times before.

Rather, this is the episode that demonstrates that Inside No 9 can pretty much do what the hell it wants: there is arguably nothing more demanding here than a battle of wits in a comfortably clichéd casing of a Dark And Stormy Night. It’s arguably the most unashamedly Tale of the Unexpected Shearsmith and Pemberton have yet delivered. The plot continually snaps back and forth, confusing and obfuscating as to what’s really going on. There are more faces to characters here than on a particularly hideous Russian Doll set.

Nina (Alexandra Roach) breaks into the rooms of university professor Squires (Pemberton) late at night. It’s not long before the latter is waving a gun in her face, and just in case any theatre students watching are tempted to make a slightly obscure joke, he’s quick to make the reference himself: a case of Educating Nina.

One of the Professor’s preoccupations is to set the university paper’s crossword, and like many crossword setters in real life, he has been assigned a name by which regular readers will recognise him and know what sort of riddle to expect – hence the ‘Sphinx’ of the title, as well as the mythical ‘What walks on four legs in the morning ..’ puzzle, which is invoked during this half hour.

A hell of a lot else happens within thirty minutes, a fair amount of which may test some more cynical audience member’s credulity. For instance, as the plot progresses, Nina and Professor Squires fill in the grid of a crossword. But what happens to them is already written within the crossword clues. Just as one might be tempted to use – well, cross words to moan about such contrivances (ought we not leave that sort of thing to Moffat?), it becomes clear that any apparent coincidences have already been earned far in advance, as long as we’ve been paying attention. This particular puzzle really is black and white.

This has all the grubby pleasure of forgotten short story at the back of a tattered copy of a Pan Horror collection, taking a relatively simple idea and spinning it out to a nastily gruesome conclusion. In that, ‘The Riddle Of The Sphinx’ has much in common with the very first Inside No.9, ‘Sardines’, where dark secrets are hinted (taunted?) at throughout, before becoming inescapable by the episode’s end.

Some hardcore fans may well demand something more tricksy, more gimmicky – indeed, less traditional. But there are still many pleasures to be had here, if you follow the clues.

‘You must have a very devious mind,’ Nina comments while Steve Pemberton regards her with a desire that could be described as hunger. ‘It has been said,’ he replies. There is much evidence of that here.

Aired at 10pm on Tuesday 28 February 2017 on BBC Two.

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