Best Interests: Sharon Horgan moves between dread, fear and anger in this heartbreaking drama

Best Interests (BBC One, Monday, 9pm) is a BBC drama about a very English couple facing an unspeakable dilemma after their daughter is placed on life support. But just under the surface it is a thoroughly Celtic affair, with Irish actors playing the three female leads and a Welshman weighing in as the tortured dad.

Sharon Horgan, Alison Oliver and Niamh Moriarty are the Irish trio. Horgan is Nicci, mother to Marnie (Moriarty, a 16-year-old newcomer from Dublin), who has a rare and life-threatening strain of muscular dystrophy. Oliver, who is from Cork, and was last seen in Conversations With Friends, completes the triptych as Marnie’s tear-awayish older sister, Katie.

Nicci is married to Andrew (Michael Sheen), a sort of flailing Everyman who does his best but, as with many middle-aged men, unravels slightly when there’s no room for dad jokes. And that is the dire scenario in which the family finds itself, as Marnie falls into a coma and the hospital concludes it is best to remove life support and let the inevitable take its course. Marnie is experiencing repeated cardiac arrests and gradual neurological damage. The ethical response, the family is told, is to end her suffering.

Andrew can see their point of view. But Nicci disagrees and, as we see in an early flash-forward, goes to court to keep Marnie alive. The scenario is unimaginably horrific, and Best Interests may upset anyone who has had a child die in similar circumstances. The linchpin is Horgan, who, drawing on her personal experience of nearly losing a daughter to meningitis, moves between dread, fear and anger.

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Best Interests is written by Jack Thorne, who has done dark family drama previously with The Virtues (starring another Irish actor, Niamh Algar). He is, however, better known for the frothier stuff: an adaptation of Philip Pullman’s preachy atheist fantasy, His Dark Materials, and of the JK Rowling stage spin-off, Harry Potter and the Cursed Child.

[ ‘I felt seen and understood’: Niamh Moriarty on starring in Best Interests ]

His new drama is slow but heartbreakingly gripping. You believe in these characters and their terrible dilemma. Should they give Marnie the peaceful death the hospital believes she deserves or have her cling on, intubated and surrounded by bleeping monitors? What a terrible question to have to answer.

Lazier television would paint the doctors as the villains. But Thorne gives Nicci an empathetic antagonist in Samantha (Noma Dumezweni), the medic who feels she has Marnie’s best interests at heart when she recommends switching off life support.

[ ‘We spent a lot of time in terrible pain’: Sharon Horgan on making new drama Best Interests ]

But she also has doubts, and her studied aloofness crumbles when alone. Marnie is fully sketched, too, in a flashback in which we see her and Katie as normal kids getting into scraps and scrapes. The four-part series is about her death, but it takes the time to celebrate her life.

There are no correct answers to the scenario presented by Best Interests. It’s just tragedy and heartache. Thorne puts it all up on the screen with the thoughtfulness and respect the subject merits.