© Johan Persson

From the outside, the Priory looks like a members' club: high walls, grand gates, lush lawns. Given its swish clientele, I'd always pictured sun loungers and smoothies, but rehab is not simply a retreat. As Duncan Macmillan's new play shows, it's a struggle.

Your room's a cell; toilet and sink in the corner. Every chair's plastic and the white-tiled walls are bare. There's nowhere to hide - not least from yourself. It's a stripped-back environment designed to strip back the self. As one of Macmillan's recovering addicts puts it: "This is as real as it gets."

Macmillan's protagonist doesn't do real. She's an actress, stunningly played by Denise Gough, who checks in as Nina - the last role she played onstage. Her real name might be Emma - or it might not be. Her brother might be dead - or he might not be. She arrives a mess, blaring down the phone, snorting coke off the reception desk, and checks herself in. The second she does so, the theatre's emergency exit signs flicker on and off.

That temptation to flee never goes away, but even if Emma stays the course, her escape route is not to fully participate. She refuses to answer questions and sits through group sessions in stubborn silence. Everything's an excuse in waiting - until a straight-talking resident (Nathaniel Martello-White) gets hold of her.

'Insightful though this is, Macmillan's play can feel like a PHSE lesson'

This is a diligent portrait of addiction, shown from within and without. Asked whether she ever blacks out, Emma replies, "I often wake up places." Time warps and, alone in her room, Emma seems to multiply: seven selves spawn from under her duvet. Gough is frankly extraordinary - and not just physically, embodying the wear and tear of abuse and the agony of detox. She's volatile, veering from fury to vulnerability, but this becomes a startlingly naked, near-the-knuckle performance: no make-up, real insecurities laid bare, emotions right at the surface.

Emma - it's not her real name - swerves between self-deception and absolute, clear-sighted honesty. You see all her shame and her guilt, the sheer scale of staying clean and the support required to do so. If the recovery process, with its group sessions and graduations, is about anything, it's about learning to help others: to listen, to help, to be depended upon. Is AA a cure, then, or is it a cult - even if its core is no longer Christianity? In any case, if it works, does it matter?

Insightful though this is, Macmillan's play can feel like a PHSE lesson. Not that it's simply tutting: 'Drugs are bad, mmmkay.' In fact, it's sympathetic to the user, acknowledging that substances can serve a function: upping the intensity of life, perhaps, or escape from its agonies. Instead, it's an illustration of the recovery process. Certain scenes - a montage of admissions and role-plays, for instance - add little to the plot and Barbara Marten's Doctor is more or less the Big Book in human form. As drama, the play's thrust is largely introspective - a (bleurgh) journey of self-discovery that's arguably more important to its characters than its audience.

The point is to make us more understanding, perhaps even more forgiving. Rehab exists behind closed doors and, seeing it like this, we can understand the effort involved and the ease of relapse. Not so, Emma's parents (Marten and Kevin McMonagle) who, in a superb penultimate scene, can't simply forgive and forget, can't start afresh or take her on trust. In fact, their refusal to do so seems like punishment - but to what end, exactly? Caitlin Moran put it best: "The world is difficult, and we are all breakable. So just be kind."

People, Place and Things runs at the National Theatre until 4 November