The historical lack of legal recourse for battered women; the persistent underreporting (and thereafter underprioritising and underconviction) of rape – not outlawed within marriage in this country, lest we forget, until less than 30 years ago; the difficulty in succeeding at tribunals and court cases involving sexual harassment or discrimination – the idea that justice is blind became risible long ago. That the blindfolded figure holding the scales atop the Old Bailey is female is cause for hollow laughter.

The Case of Sally Challen (BBC Two) is a 90-minute film documenting the appeal of the eponymous 65-year-old who, after 31 years of marriage, killed her husband Richard with 20 blows from a hammer as he sat at the dinner table one night nine years ago. She was sentenced to life imprisonment with a minimum tariff of 22 years.

The film recapped the original trial. Friends – of Richard, of Sally and of both – remembered a lively, gregarious pair: a successful man, a caring, loving wife and mother. There were occasional cracks in the facade, but no marriage is perfect, is it? Then things started to fall apart as Sally became aware of Richard’s serial infidelities. She started drinking more and obsessively tracking his movements, and Richard was revealed to be a customer at a local brothel raided by police. The good burghers of Claygate, Surrey, did turn against him then. And they could see Sally was suffering. But they never saw murder coming.

Nor did her family. Not her many siblings, who remember how much fun Sally and Richard had together when she first met him when he was 22, and she was nearly 16. Nor her two sons. James has lent his mother public support, but finds it too upsetting, his partner explains, to talk on camera. But David is an eloquent contributor throughout, even though he has clearly been through almost unimaginable trauma.

The prosecution had plenty to work with. The defence seemed to have very little. They chose not to bring up Richard’s abusive behaviour towards Sally, in case blaming the murder victim turned the jury against her. The full extent of the injustice only became clear when Challen contacted the human rights lawyer Harriet Wistrich, and her cause was taken up by Justice for Women, which specialises in supporting women prosecuted for killing violent male partners. Challen furnished the organisation with a full account: Richard had raped her many times, endlessly humiliated her and constrained her freedoms in a million ways, large and small.

Wistrich and her team began to put together what would become a landmark case, arguing that Challen had been the subject of coercive control. In the years since Challen’s trial, the nonphysical means abusers use to instil fear in their victims, to erode their sense of agency, worth and esteem, and to bind them with invisible chains, had become recognised as an offence.

It remains astonishing to me that it took until 2015 for the law to recognise a phenomenon that has been an everyday part of women’s lives since time immemorial. Even if you have never found yourself circumscribing your behaviour with a partner to keep the peace; even if you haven’t experienced it directly, you will almost certainly have done so indirectly. You’ll have a friend you keep a close eye on because her husband speaks to her so unpleasantly, because she defers to him more than is natural or because there’s just something off about their relationship. And you know that what you see in public is only the tip of an iceberg.

But until Wistrich and Women for Justice made it part of Challen’s appeal trial, it had not been properly tested in court. You can see the strain and responsibility etched ever deeper on Wistrich’s face as the trial approaches.

As you doubtless know from the headlines, they were successful. Challen, having served nearly nine years of her sentence, was freed. Some see her release as opening the floodgates for women to kill with impunity. Others see it as the start of addressing a deep historical failure of understanding and of lawmakers to protect that half of the population who have always needed it most.

The film delved deeper than most into the intricacies of the case, and the main players’ psyches, tracing the filaments of each without becoming either tangled up or intrusive. And it resisted the temptation to make Richard into a monster, staying with the far more uncomfortable truth that abusers create the relationships and situations that suit them by being all too human in their nuanced reading of people, their needs and vulnerabilities.

Challen said little other than to read aloud her formal statement to illuminate various points of the film. Until the end, when she stood in the church she goes to “to think about what happened to Richard. And how I still miss him.” Invisible chains.

If you or anyone you know has been affected by domestic abuse, visit loverespect.co.uk or womensaid.org.uk for advice and support