"Where are you off to?" my colleagues asked. To see a funny thing, I replied. You go into a little room and get a personal performance just for you. "Soho, then," they said. Well, actually, it was the Barbican. The Pit was its usual, inertly hot self, but nothing steamy was going on. Helium was the latest effort by Slung Low, an award-winning team of young artists with all sorts of backgrounds, from film and dance to fiction. Their intention, it said in the programme, was to use "all the technology and resources available to the 21st-century artist to attempt that oldest of artistic aims - a good story well told". That rather undersold their ambition.
Actors and directors have always longed to breach the "fourth" wall, the invisible one that separates a performance from its audience. You might argue that, when drama really works, no special measures are needed: you completely enter the story and don't need the house lights turned up or to be cajoled into minor acts of participation. But I am generally with Dr Johnson who said - although for the life of me I cannot find where - that no theatre-goer, however entranced, forgets he is sitting in a theatre. Attempts to evade that truth can be silly - and they can be rewarding.
By far the best piece of theatre I saw last year was Punchdrunk's version of Poe's "Masque of the Red Death", which transformed the Battersea Arts Centre in London into a spooky labyrinth of Victorian Gothic, each of its rooms playing out a scene from the story. On the way out, somehow, the audience, which had walked round randomly, alone or in pairs, was caught up in a swell and arrived in a ballroom where a brilliant danse macabre was performed.
Helium is not quite in that league, but I won't forget it either. My bespoke performance lasted 15 minutes, and was indeed "a short adventure for one". I was greeted by a young woman who briefly set up a story about a teenage girl called Bella whose grandfather, Max, was dying. He had, mysteriously, sent her a white helium balloon each birthday. She needed to know more.
Lucy led me to the gate of five rooms. I was to enter, close the door behind me and observe. The first was Max's study, where Bella had elected to stay examining Max's books and photographs while her mother waited at the hospital. Among the documents were drawings of a gargoyle. In the background, a radio crackled out a distant conversation about memory between Max and this fiend. The next room contained a beautiful miniature replica of a music hall, where a film was shown of Max as a boy being traumatised by a memory reader. After that I entered a bomber flying over a Germany city that the young Max was helping destroy. This was where he formally meet his gargoyle, an ornament from a firebombed cathedral.
It is, I gathered, this act that haunts Max and is purged in part by the innocence of his granddaughter. The penultimate room is the hospital ward where, invisible to us now, Max talks to the grotesque, who has assumed the persona of his nurse. Finally, I found myself in a white cubicle with no actors in it - only the voices of Max and his daemon, and colours, suggesting a cathedral. As I left, I was given a box containing (no prize for guessing) a white helium balloon. More usefully, there was a programme and a short story amplifying what I had just seen.
The actors' work here was terrific, at once corporeal and ghostly, and the designs even better. There were some strange errors of judgement, however. The actor playing the nurse/gargoyle was dressed and painted in black, giving the unfortunate appearance of a vaudeville minstrel. Patrick Stewart as the disembodied voice of Max failed to inject any fear or frailty into his actorish performance. And at the end, when they waved me goodbye, the cast reminded me more of Teletubbies than offending spirits.
My serious reservation, however, was that the generally brilliant realisation was wasted on a portentous, moralistic, woozy piece of writing redolent of a Radio 4 afternoon play. But I forgive Slung Low, for it was only on the way home I reached that conclusion. During my adventure for one, I was as stimulated by it as by anything the fleshpots of Soho might have offered.
Ends 20 September. Bookings: www.barbican.org.uk 
Andrew Billen is a staff writer for the Times
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