Best Interests, BBC One review - a family feels the unbearable strain of terminal illness

This is possibly not ideal viewing for a spell of sunny weather in June, but Jack Thorne’s drama about a family trying to cope with a terminally ill child is as compelling as it’s painful. Sharon Horgan and Michael Sheen star as parents Andrew and Nicci, and Best Interests probes their private agony in piercingly intimate detail, but the focus also pulls out to encompass prickly issues of ethics, morality and the labyrinthine innards of the NHS.

It might have ended up as a preachy, finger-pointing litany of sclerotic bureaucracy and entrenched attitudes, but Thorne’s writing and an excellent cast ensure that the drama is always grounded in the personal. Sheen and Horgan are consistently convincing as a couple whose natural closeness comes under increasing stress as the saga of their daughter Marnie’s condition unfolds. The terror of their meeting with the specialist who calmly spells out his diagnosis of muscular dystrophy and its relentless progress (Marnie won’t be able to walk unaided, she’ll suffer progressive degradation of her major organs etc) is enough to freeze the blood, culminating with his ominous comment that “her life may not be a full one.”

Despite her parents’ highest hopes, Marnie’s life becomes a grim litany of crises, with cardiac arrests and seizures rendering her predicament increasingly perilous. But while Andrew watches events unfold with a mixture of horror and resignation, Nicci’s fighting instincts increasingly come to the fore. She’s convinced the comatose Marnie can hear and understand her, and she won’t accept that the situation is becoming hopeless.The viewer’s sympathies are also liable to be pulled in different directions. Obviously Nicci is prepared to do anything to save her child, but will it ultimately be in her – as the title would have it – best interests? Samantha (Noma Dumezweni), the doctor in charge of Marnie’s treatment, seems like a sympathetic and experienced professional, but Nicci can’t accept her view that the time may be approaching when further treatment of Marnie will be futile and perhaps cruel to the patient. When Nicci gets in touch with a group called Every Christian Life who suggest that the huge costs of paediatric intensive care mean that the NHS has to pick winners and give up on the least hopeful cases, Nicci goes into full-scale crusading mode to make sure nobody is allowed to turn off life support for Marnie.

Inevitably this has wider repercussions. Marnie’s older sister Katie (Alison Oliver) delivers a beautifully nuanced performance as a teenager still struggling to find her own place in the world, and now having her emotions twisted and torn by her sister’s illness and the rift that the situation has suddenly opened up in her parents’ relationship. Flashbacks to happier times with the sweet, gently uncomplaining Marnie (Niamh Moriarty, pictured above with Alison Oliver) are almost too poignant to watch. Her friend and potential partner Hannah (Mica Ricketts) tries to get Katie to loosen up and party down, but the fact that they’re using a pilfered supply of Temazepam doesn’t look good when it hits social media.

This isn’t a barrel of laughs, but Thorne lands his punches with ferocious accuracy. It’s television that lingers in the mind.

• Best Interests continues on BBC One on 13, 19 and 20 June and is on BBC iPlayer
• More TV reviews on theartsdesk