Stacy, by Jack Thorne (Skins, Cast-offs, This is England '86 and This Is England '88), is a riveting one man show – a characteristic mix of very funny and painfully sad – that manages to say a lot without seeming to. It centres on twenty-something-year-old Rob, played brilliantly by Tim Dorsett, who tells us about the events that happened to him over a couple of confusing days, in a narrative intercut with a patchy recollection of events from his childhood. Dorsett brings Thorne's script alive perfectly.

The stage is bare apart from a chair and a projector which splashes images on the wall behind Rob, clicking with each new picture like a crime scene investigation camera. The projector's limited repertoire of people and places responds to what Rob is talking about, so that, when Stacy or her flatmate Shona are mentioned, a low-res image looking like it's been picked at random from a Facebook page snaps up. At other times the slide show is derailed by an image that doesn't quite fit – or by something disgusting and best kept at bay, not plastered up for the world to see. Without giving too much away, Stacy is about intimacy and what we do with it; about relationships and the ordinary moments of trauma that leave their imprints on us, and about being exposed. Like Rob, the slides sometimes say more than they should and veer in unexpected directions, and there is a sense in which the pictures – like his own recollection – are not quite up to scratch.

Stacy is directed by Sam Miller, whose theatre company allthepigs (which describes itself slightly weirdly as “theatre for the audience”) is hosting a First-Time-Writers' Initiative alongside the production. This means that before every show there is a staging of a play by a writer new to writing for the stage: so you get two plays for the price of one. I saw The Stolen Inches by Cordelia O'Neill. The set-up is a nice idea, but given that every night is a different play of a different length (this one was an hour long – much longer than front of house had anticipated) and is inevitably filled with the friends and family of the new writer and the for-one-night-only cast, Dorsett is up against it. Rob is a naturalistic but complex character and sustaining an hour and ten minutes of monologue is difficult. Hanging about backstage for ages beforehand, not knowing when you are going to go on, must be hard work.

I mention this mainly to highlight what a great job Dorsett does of filling the black box space with lively energy. Apart from anything else, his performance is an exceptional feat of learning. He gave the impression of being so relaxed that when some moron got up to go the loo, crossing the New Diorama's small stage a foot away from where he was performing, he glanced slightly curiously at him and immediately regained his flow. It is difficult stuff, but Dorsett gets the measure of Rob exactly: every throw-away gesture is exceptionally well-judged – just as the there's not a word out of place in the script.

What makes Stacy such a good play is how much is left unsaid, so it seems a shame to go through it and spell out the themes. Thorne's mix of specifics and vagaries is realistic: Rob describes thoughts and scenes that are on the edge of observation, layering images and memories in completely believable patterns and cycles of recollection. If you are interested in other people and why they are like they are, this is a must – a real Fringe gem.

Stacey, at New Diorama TheatreBecky Brewis reviews Stacey by Jack Thorne at the New Diorama Theatre.5