‘Best Interests’ Is a Deeply Empathetic British Series

There are no villains in “Best Interests,” a heartbreaking limited series that arrives on Acorn TV on Monday. Instead, the series, a four-part British drama starring Sharon Horgan and Michael Sheen, is about two people in impossible circumstances who are trying to do what they think is right.

The episodes follow Nicci (Horgan) and Andrew (Sheen), a married couple whose daughter Marnie (Niamh Moriarty) has a form of muscular dystrophy. Early on, the cheerful Marnie ends up in the hospital with an infection that leads ultimately to brain damage. Her doctor (Noma Dumezweni) recommends stopping treatment.

This is where Nicci and Andrew, whom we immediately understand to be dedicated parents, diverge. Andrew looks at his child and believes the girl he once knew is gone; Nicci sees a callous system that wants her disabled daughter to die. The writer Jack Thorne, known for “His Dark Materials,” never allows one side to be the “right” one. Horgan’s passion convinces you there is a chance for Marnie; Sheen’s despair makes you believe there isn’t.

In the middle there is Nicci and Andrew’s other daughter, Katie, played by Alison Oliver of “Conversations With Friends.” Katie is a teen who has always existed in the shadow of her high-needs sister. She copes by sneaking cigarettes and wants desperately to appease both her parents. While Oliver portrays Katie’s pain well, her story ends up being the weakest because of an ill-advised plotline involving a bad girlfriend and the theft of Marnie’s unused drugs. It is the most outlandish the series gets.

“Best Interests” is at its most fascinating, though, when it invests in the emotional compromises all these people make as they try to fight for Marnie. Andrew is shocked, for instance, that Nicci would align herself with a Christian organization, which is likely anti-abortion, in order to pursue a court case against the hospital. Nicci, on the other hand, sees Andrew’s resistance as abandonment.

Horgan and Sheen propel the show with their wonderfully complicated performances. Horgan, best known for sharp-edged comedies like “Catastrophe” and “Bad Sisters,” brings wry humor to Nicci even in her character’s darkest moments. But she also depicts the unimaginable agony of a parent in limbo. In Sheen’s dejected, empathetic depiction of Andrew, you see how crushed he is by the notion that Marnie is already gone.

Sadly, the voice that is missing is Marnie’s. Her life is rendered through flashbacks that feel like rosy, one-dimensional glimpses of what once was. But as a depiction of what happens once she can no longer speak for herself, “Best Interests” is devastatingly complex.