AGED eight, Gracie is a loved and lovable child, arriving in a new community where there are bicycles and dolls and friendship. At 15, she is on the verge of womanhood, trembling to find the courage to set herself free from conformity with everything she knows.

Joan MacLeod is an award-winning Canadian writer, and her one-woman play Gracie is set in the Fundamentalist Church of Jesus Christ of the Latter-day Saints in British Columbia.

Gracie’s mother has taken her three daughters and a son to join the community, where she is to become the 18th wife of one of the elders. Almost immediately, Gracie’s older sister is married to a much older man, and a baby is born.

Seen through the eyes of an eight-year-old, it is a community of love, purpose, and the blessing of God. But Carla Langley’s superb performance reveals how, over seven years, her perception slowly changes, and arranged marriage at a young age seems to verge on abuse.

I use nuanced language to describe what might be seen as abhorrent, because that is typical of MacLeod’s compelling script. The community is a place of care and security. It’s “a family that loves God and loves each other”.

As an audience we are placed among those who criticise a world we hardly know: “They don’t like the way we live even though they know nothing about us.” (That line made me sit up, because the same might be said of the way a Church of England congregation is viewed by many.)

But it is also a place where women have scant control over their lives, children bear children, and those who leave (like Gracie’s brother Billy, who rejects the community, escapes, and returns covered in tattoos) are made a pariah. And where is God in all this? “God doesn’t talk to girls. But he listens. I pray for God to tell Mr Shelby to find me a husband who’s sweet and kind and not too old.”

Gemma Aked-Priestley directs the piece fluently, using every inch of Bex Kemp’s tiny set. The simple triangular platform hints at a mountain, a chest, a cell, and, in the play’s closing moments, a changing-room in which Gracie struggles out of her prairie frock and dares to try on modern clothes.

The production stands or falls on Carla Langley’s 90-minute performance. Oh boy, does it stand! As the young Gracie, she is delightful without being cute. As a teenager, she makes us ache for her to dare to step into freedom. Her triumph is to make this play more universal than a story about extreme, polygamous Mormons. It leaves us reflecting on the cost that anyone who challenges the culture into which he or she was born will face.

Gracie continues at the Finborough Theatre, 118 Finborough Road, London SW10, until 15 May. It plays on Sundays, Mondays, and Tuesdays. Tickets from finboroughtheatre.co.uk or phone 01223 357851. Our reviewer attended a preview performance.