The Good Dad is always looking out for the family, and with his weak heart they never want to do anything to upset him. But he tells Donna that she is his special one, gradually separating her from her twin, Carol, and leading to abuse that escalates over time into something horrifying behind a family that seems so normal.

Gail Louw’s play shows how the instinct to protect that normalcy and not rock the boat can, in the hands of a master manipulator, be perverted into protecting an abuser at the expense of all those around him. Donna is in prison now, rejected by her mother and twin sister, and all three have their own take on what led to this and why no-one stepped in.

None of their reasons seem quite good enough – no-one wants to disrupt the family, it’s because of Dad’s weak heart, because he’s a good dad, because they won’t be believed – so the success of the play lies in Louw’s ability to show how abuse is allowed to spiral, as no-one can acknowledge even to themselves what they know what is going on.

Sarah Lawrie’s tremendous performance illuminates the complex emotions at play here. The three characters have to be distinctive even though they are a mother and identical twin daughters, and she hits the degree of differentiation perfectly. But what the three share is brittleness and defensiveness. They blame each other – and even now Donna, despite the abuse, still finds satisfaction in being the one who was chosen over her sister.

Lawrie also incorporates elements of physical theatre that unobtrusively create moments of magic, such as the use a red towel during a depiction of childbirth which turns into the baby nestled in her arms.

There is some unevenness in the sharing of perspectives. Perhaps inevitably Donna dominates, but I would have liked to have seen more of her twin Carole; the damage done to her is less obvious, but it’s hinted at by Donna and her mum and will be equally long-lasting. It would have been good to see that explored further.

Of course, it is an uncomfortable watch. But in making a horrifying and unnatural course of events seem almost inevitable, the play succeeds in showing, if not why these things happen, at least how they do.