Diagnosed is a modest piece of theatre: clocking in at just 40 minutes, a single scene, presenting one conversation between two actors. At first glance, it's a typical rom-com – the kind where she's a bit kooky, he's a bit grumpy, but she'll slowly win him over and they'll be in love by the end. But it's not that simple. There are secret depths to the storyline, and some genuinely difficult questions brought out through the combination of compelling writing and impeccable delivery.

The tale is set in a hospital waiting room, where Ryan – played by Matt Anderson, who also wrote the script – is waiting in his pyjamas. It emerges soon enough that Ryan's not the one who's ill; someone close to him is lying in the ward, and the news is grim. Into this bubble of pain bursts Ellen, also there for difficult reasons, and expressing her nerves through terrible humour. Audrey Roberts-Laverty brings an appealing mix of restlessness and audacity to the role, as this self-described "irksome stranger" does all she can to wind the scowling Ryan up.

The scenario's instantly recognisable and, while the plot requires Ellen to be a little larger than life, Ryan is utterly believable. Introverted and taciturn, restrained in a very particular way, he's halfway towards being a gentle giant – the kind of man who (as Ellen observes) "doesn't like to argue". Ellen, of course, won't leave it there, and there's a lot of laugh-out-loud humour as she gently teases Ryan out of his shell. Her antics stay just the right side of being too annoying to bear – and although her insistent questioning is sometimes very clumsy, she obviously has Ryan's wellbeing at heart.

The conversation heads off in some unexpected philosophical directions, but Anderson's script treads lightly enough to avoid any sense of labouring. His direction – aided by Amber Johnston – helps as well, filling the space with natural movement and maintaining enough visual interest to balance the wordy dialogue. There's a genuine sweetness to the way these two very different characters reach out to each other, and for a while it looks like we're on our way to a classically honeyed finale.

But we can sense that there's something we're not being told… and when the truth comes out, it's both startling and morally troublesome. Perhaps the timing's a little wonky here – the play ends before we've really had a chance to process what we've learned – and for me, the twist came too much out of the blue, lacking that satisfying feeling that we could have seen it coming if we'd paid more attention earlier on. But Anderson successfully poses an awkward question with no obviously correct answer, and offers something to debate or think about long after the actors leave the stage.

Diagnosed feels a little truncated, as if – after an excellent build-up – it's missing a postscript or final scene. But with just the tiniest bit of development, this will stand up as an entertaining, credible and meaningful full-length show. I hope it has a life after the Edinburgh Fringe, and I'll certainly watch out for anything this talented group do in the future.