Writer/actor Samara MacLaren's chilling monologue, thoughtfully directed by Marilyn Imrie, and acted by MacLaren herself, traces the life of 14-year-old Nina, whose life is irrevocably changed when her former best friend Ailsa Benson is reported missing in school assembly.

As a high court judge in the present day, Nina is an outspoken and confident high achiever. But by simply donning a pink hoodie and widening her eyes, she's her teen self: a torrent of innocent self-absorption and hormones, who is nonetheless articulate and likeable. The line, 'I read a sonnet and tried to cry', really sums up her adolescent need to be poetic, special and angst-ridden.

The slow drip-feed of information, particularly when Nina's imagination starts to work overtime, is beautifully wrought. MacLaren's writing is never heavy-handed nor clichéd, finding truth in the forensic details of girlhood and its vulnerability.

It's a character study with real bite, meditating on the male gaze (especially towards young girls), the need in kids to grow up too early, and issues around trust when families are divided by parental separation.

And as MacLaren – five months pregnant and patting her stomach – delivers the final killer blows, her words resonate like a quickening pulse. Ailsa Benson is not an easy or reassuring experience, but a thought-provoking and haunting piece which lingers long after it's finished.

Assembly Rooms, until 25 (not 13) 2.20 pm, £10–£11 (£9–£10).