Kids' TV in the 70's was creepier than it is today. Real-world series like Children of the Stones are noted for their quality – but also for the incongruous darkness of their themes. So Hermetic Arts' Unburied grows on fertile ground; an homage to this vanished strand of junior horror, it's built around the story of a lost children's series, filmed for ITV but strangely never shown. It's not so much a play as a live investigation, as actor Carrie Marx delves into the archives and inevitably unearths more than she had planned.

Unburied isn’t the first Fringe show ever to have pulled this stunt, but it's a particularly well-worked and convincing example. Ostensibly appearing as herself, Marx talks directly to the audience; we're at a live recording of her podcast, she explains, listening in as she reveals the next stage of her research. The podcast is a clever gambit, licensing Marx to drop in chunks of lyrical musing, yet enabling her to "pause the recording" whenever a more direct connection with the audience is required.

It's as obvious as it could be that a spooky shock is coming, but the nature of that horror stays obscure until the end. As Marx describes her research, she builds an intriguing and suspenseful mystery – a finely-honed puzzle which of course, I will not spoil. I will say, though, that there are no bangs or crashes or descents into screaming madness; the story is subtler than that, and creepier too. If you're on the ball you'll spot some disturbing patterns a few moments before Marx does, and I had genuine goosebumps as the carefully-planted clues clicked ominously into place.

Cleverly, the story melds familiar ghost tropes with the trappings of modern life; just as a turn-of-the-century reader might urge the hero not to open a forgotten tome, so I wanted to scream – for the love of God – don't type that into Google. The seminar format and contemporary references add up to a truly immersive piece, and snapshots from web pages flash up on a screen in the background. You could check them afterwards, of course, and find out if they are real – but I think it's more fun not to. If they're mock-ups, they're impressively believable; if they're genuine, it's equally admirable how seamlessly they've worked these materials into the plot.

And the story deals with contemporary concerns too, though it waits patiently before it shows the strength of that hand. An early theme of nostalgia for our childhood develops into something more concerning, while a thought-provoking message about political engagement is delicately woven in. For a while I worried there were just too many hares running, but the last scene proved me wrong: it all comes together, naturally but decisively, and there's a lot of pleasure to be had in thinking back on the meaning of seemingly random asides.

The opening is surprisingly unmemorable – a little too much like a genuine PowerPoint seminar – and Marx's character could perhaps do with some mannerisms to lend interest to those first scenes. But the dynamism grows as the plot accelerates, and the cleverness of the conclusion thoroughly justifies all five stars. Unburied is a masterpiece of alternative storytelling, a work of theatre so convincing you might have imagined it's real. But the style is complemented by genuine substance - and the message it delivers is an urgent one for our day.