Set in the American Deep South, Proxy is a disturbing yet enthralling solo show – which tackles a complex topic, and trusts its audience to search for the truth amidst a bundle of unreliable tales. Written and performed by Caroline Burns Cooke, it's a sometimes-fragmented but always-engaging monologue, which slowly and inexorably paints a vivid picture of motherly love gone terribly wrong.

The title refers to Munchausen syndrome by proxy – a controversial psychological condition, which not everyone agrees actually exists. But instead of debating the medical technicalities, Proxy tells a specific and entirely believable story: of a girl called Gypsy and a mother called Dee Dee, who genuinely thinks that she has her daughter's best interests at heart. If your child has a problem, and a little operation could correct it, wouldn't you fight tooth and nail to make sure it went ahead? Of course you would – that is, until you find out what the "little operation" actually entails.

The power of Proxy doesn't lie in the horror it describes, but in the gentle way it descends, step by tiny step, into the darkness. Each of Dee Dee's individual deeds seems logical enough, the action of a caring and protective mother. But slowly, you realise how far we've strayed from safety, how out-of-control her imagination has become. A casual word here, a telling self-justification there, add up to a haunting picture of love that's turned to abuse – all in the clear but uncomprehending sight of professionals, neighbours, even the media.

And yet, there's room for sympathy. Burns Cooke's freewheeling, folksy monologue is full of rambling asides, which flesh out Dee Dee's character and make it all but impossible to view her story with a callous heart. We learn that her own mother hurt her, and understand how her wild over-correction is hurting her own daughter in a very different way. We realise that the world at large, through its Disneyfication of Gypsy's story, unwittingly feeds the abuse. And we see that she loves her child – wholly, unselfishly, beautifully – the precious girl she heart-rendingly describes as "my tiny little daughter, no bigger than my thumb".

Though most of the story is told by Dee Dee, we also hear from her daughter Gypsy, and from a third character – an outside observer – whose role only later becomes clear. The characters are subtly delineated, marked out primarily by small changes in costume; for the first few minutes it's far from clear who you're hearing from, and why. Don’t worry though: that's completely intentional, a gambit which mirrors the confused web of half-truth and falsehood we hear as the story unfolds. By the end, you'll not only know what's happened, but exactly how Dee Dee and Gypsy arrived there.

The ultimate consequences verge on the grotesque and, to my mind, jar just a little with the disquieting subtlety which has come before. But only a little; it's a believable conclusion, and makes a new and valuable point about what finally must happen if the truth is suppressed for too long. Proxy is a masterpiece of performance, a triumph of layered character-building, and a sensitive but alarming depiction of an all-too-realistic decline. It doesn't shy away from the shame of what Dee Dee's done, but it does invite us to understand her.