In the opening minutes of Best Interests (Monday and Tuesday), an estranged middle-aged couple made their separate ways to court, pausing outside it to look at each other with a mixture of furious reproach and overwhelming regret. From there we cut to a scene that perhaps overdid the evocation of Happier Times as the same pair laughed endlessly together on a train, before nipping off to the toilet for a spot of giggly conjugal naughtiness. Once they got home and picked up their two daughters from a neighbour, they soon showed what terrific and loving parents they were too – not least to 11-year-old Marnie, whose muscular dystrophy meant she needed especial care.

But then, just as husband Andrew (Michael Sheen) was celebrating the end of a happy day with some Stone Roses and a joint, Marnie suddenly developed an infection and needed the latest of her many trips to hospital. There was, however, one big difference from her previous visits: this time she didn’t get better. Instead, after several weeks of unavailing treatment, her doctor Samantha ominously introduced Marnie’s parents to a palliative-care consultant and recommended that they seriously consider allowing her to die – the alternative being a lot more pain, a lot more brain damage and no realistic chance of recovery.

Even so, mother Nicci (Sharon Horgan) was having none of it. Holding on tight to her mantra about Marnie being a fighter (‘She’s wilful’), she refused to accept the verdict of the doctor, the hospital’s ethics board or the mediator brought in, as Nicci saw it (possibly rightly), to rubber-stamp her daughter’s medically decided fate.

Best Interests is written by Jack Thorne, whose last TV drama was Then Barbara Met Alan: a heartfelt but off-puttingly shrill piece of agitprop about disability rights. Happily, the contrast here couldn’t be greater. Nicci clearly sees herself as a lone fighter against NHS ableism, as well as for her daughter’s life – but the programme itself never allows us so simple or comforting an interpretation. Of course, Nicci’s sense of denial is entirely understandable. Yet could the chair of the ethics board be right to say that ‘It’s impossible for parents to make this decision’? Could Nicci be just condemning her comatose child to more pain?

At times, in fact, Thorne seemed to go so far as to rather bravely suggest that she might not merely be wrong, but even the inadvertent villain of the piece. After all, her irrationality may be pretty understandable too, but that doesn’t make it any less irrational. It also leads her to lash out wildly in all directions – particularly her blameless husband’s. ‘She’s always been a thorough and kind doctor,’ Andrew said of Samantha at one point. ‘And you’ve always fancied her,’ Nicci angrily replied. When he eventually plucked up the courage to wonder if they should maybe spare Marnie more suffering, Nicci accused him of having always wanted his daughter to be dead. (Neither of these assertions, Thorne made clear, was remotely true.)

And yet, as it’s now transpiring, the idea of Nicci purely as someone driven into damaging delusion by grief might also be too simple. Towards the end of Tuesday’s second episode, she consulted a lawyer and learned of several real-life precedents for parents successfully fighting in court for their children to be treated further – and apparently being proved right. Visiting a local mother who went along with the doctors’ advice and allowed her disabled son to die, she discovered a woman full of articulately expressed remorse.

But if this is making Best Interests sound like a gripping Shavian think piece, that would only be half-right – because it also packs a ferocious emotional punch. Indeed, you could see the programme as four separate, beautifully acted and equally powerful tragedies – for Marnie and her inevitably neglected older sister Katie, as well as for her parents: tragedies that are all the more tragic precisely for being so separate.

While Marnie lay in her hospital bed, we got several heartbreaking flashbacks showing how much she once enjoyed her life (and how great the disabled child actress Niamh Moriarty is at playing her). But in their way, the rest of the family are now just as isolated from each other as she is from them. Love, it turns out, is not enough, with none of them able to emerge from their private grief and make proper contact. Not only can they not share one another’s feelings, but they don’t have enough emotional room left over to even acknowledge them.

The final two episodes of Best Interests come next Monday and Tuesday. If the programme manages to hold its nerve – i.e., continue to honour its commitment to the utter intractability of the situation – the result could well be one of the best (although not jolliest) TV dramas of the year.