The Honourable Woman is hardly a light-hearted comedy romp at the best of times, but the sixth episode – which opens with a rape and ends with a young boy shooting himself – is bleaker than a Dartmoor funeral.

Like a spindle of black cotton unravelling, ‘The Mother Line’ is dark from the first and stays that way: gruelling television that demands more of its audience than any previous instalment. Yet the viewer’s reward for not flinching is the most compelling episode of the series yet.

The looser strands of story are pulled together like the cords of a drawstring bag and the pace increases almost insidiously, like a driver who doesn’t notice the speed of his car until he realises it’s going too fast to avoid a collision. This is a tightly-plotted ride through the night with no seatbelts, no lights and (it seems) very little hope.

Things have certainly never seemed so hopeless for Nessa Stein (Maggie Gyllenhaal). Despite the horrors that have piled in front of her since childhood, when her father was stabbed to death in front of her, yet she has always found the will to continue. Not even the grim events in Gaza, when she was kidnapped and brutally assaulted by the son of her father’s killer, were enough to crush her. But now she seems at her lowest ebb.

She is raped again – not as part of a grim terrorist plot but by an ordinary, anonymous scumbag, a sentient shit in the skin of a man – and left wandering the street, dazed and bleeding. Sir Hugh Hayden-Hoyle of MI5 takes her in and offers to have her attacker charged, but Nessa refuses. Later, the dishevelled spy reveals he knows about her previous assault – and about the sleaze within the Stein Group.

‘Our institution’s not corrupt,’ Nessa protests, but the spindle is unravelling fast now. Soon, two more galling truths are clear: to get her secret son Kasim back safely from the kidnappers, she must award the lucrative third phase cabling contract to Jalal El-Amin (Raad Rawi); and the sniffer device intercepting Palestinian communications is sending information back to the Stein-funded university in Tel Aviv. (The listening post in the basement is still operational.)

This venal trail leads in only one direction: brother Ephra (Andrew Buchan). ‘You’ve ruined us!’ she wails, tottering on the brink of insane despair. ‘You wouldn’t be here now if it wasn’t for me!’ Ephra snaps back, full of self-righteous rage. It’s a fierce scene, fizzing with anger and recrimination – and if Buchan goes slightly over-the-top, Gyllenhaal is pitch perfect on every line, neither accent nor intensity wavering for an instant.

Stephen Rea is similarly impressive throughout – although Hayden-Hoyle’s wry dejection is a continent away from the Steins’ emotional extremes. (‘Pretty soon someone’s going to bowl me a googly and I’ll be out,’ he muses. ‘It’s cricket … a cricket analogy. I don’t know why I said it. I don’t even play the game.’) There’s also an unsettling performance from Philip Arditti as Saleh Al-Zahid, the man who raped Nessa in Gaza and received a scarred fireguard of a face in return.

Al-Zahid is now in Britain on a killing mission – the details of which remain tantalisingly murky. Staying with a reluctantly hospitable family, he speaks with their youngest son of the terrible things he has done: ‘I didn’t want any of it. [But] a soldier has no choice.’ Later, he returns to the house to find the kid lying on the floor with a bullet in his chest.

‘Your boy is strong,’ he tells the grief-stricken father. ‘He will survive. But I was never here.’ The political cause is paramount and human life is expendable. With that, Al-Zahid is gone. In two episodes’ time, The Honourable Woman will be, too. And that’s the bleakest thing of all.

Airs at 9pm on Thursday 7 August 2014 on BBC Two.

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