Episode 4 of What Remains is a crafty torture that preys upon our Sunday night lethargy and desire for a conclusion, and uses our revulsion of every resident to mislead us. Of course it makes sense when Joe Sellers admits to the murder. It makes even more sense when we find out it’s Liz. But then we discover it’s not Liz. Twist after twist, a Matryoshka Doll of motives and suspects pops open until at its core is the truth: not a premeditated event but – just as in Broadchurch – hands that held a neck too tightly in a moment of anger.

Peggy’s hands. Oh Peggy (trembling maestro Victoria Hamilton). One half of a relationship so toxic that it’s a wonder it didn’t set the carbon monoxide alarms off. Chances are you didn’t suspect her. You were so busy balking at her being mentally abused by Elaine that you don’t stop to wonder why she was trying to take her own life, or why Elaine was keeping her prisoner in the first place. It was Elaine protecting herself, keeping the human evidence under lock and key. Limiting the damage. That’s what What Remains has been about for every character: trying to hide the emotional and physical wreckage.

And though it’s Peggy who’s the killer, it’s Elaine who comes off as the real villain when the final ten minutes swerve from sedate whodunnit into taut ‘the killer’s in the house!’ horror. Indira Varma sinuously moves from cold-hearted bitch to cold-blooded killer, and you might level the criticism that she’s almost ludicrously competent at it if you weren’t so preoccupied with worry for Len and Vidya and her baby.

For three weeks we’ve wondered what the significance of the bow and arrow was (Metaphor? Whimsy? Does Tony Basgallop like Robin Hood as much as he hates pineapples?) when it is in fact the archery version of Chekhov’s Gun: once the bow and arrow is introduced, it’s only a matter of time until it’s fired at someone. And as it thuds into Elaine’s chest you can’t help but cheer.

And then that cheer deflates into realisation. You’ve been hit too. You don’t want lovely Len Harper to die but you know he probably will, especially as the ‘click’ of the timed light going off – symbolic of death in the first episode – snaps shut the story. Cruel as it is, it’s dramatically perfect, and while we adore David Threlfall we’d feel cheated of a good end if Len Harper was to return. It’d be like Hamlet waking up and solving crimes (hmm, memo to self…).

What Remains has been stunningly good drama, and as is always the case with the best TV, it’s been an ensemble effort. Not simply of the acting talent, but of direction, production, sound, editing, and all those many unnoticed important jobs that whizz by us in the credits. Rounds of applause for all, please. And one for Tony Basgallop, who just struck us in the heart and punched us in the gut. Hopefully he’ll be clutching some much deserved awards for his expert aim.

Aired at 9pm on Sunday 15 September 2013 on BBC One.

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