I received an email shortly before noon. The content of the email was brief but informative - most importantly it finally revealed where I needed to be that evening. There's also a single instruction that says, "please try to look inconspicuous". Later that evening I'll be reminded of the need to blend into the crowd and not draw attention to myself or the others there. The secrecy is exciting, but also slightly disconcerting, especially when I'm told that what we're doing isn't "technically illegal". This ambiguity is where Monument really shines, a theatrical happening that distorts your sense of reality and truth.

Created by David Rosenberg and Ben & Max Ringham, Monument takes its audience to secret locations in Central London and drops them into a world of bizarre and touching vignettes that hint at the multitude of stories we pass by every day. Unlike other immersive, or indeed public, performance pieces, Monument isolates its audience members. Attendees hear the performance audio through an app, with the use of headphones (particularly closed-back type) essential. The effect of this is to deliver a disorientating aural experience, with the real sounds of London blurring with the fictitious sounds of the performance. The fact that the app that powers the experience can be installed on any smartphone, and the audience use their own headphones means that identifying fellow audience members is difficult, with any headphone adorned citizen a potential sharer of this secret.

Monument plays into this, using sound design and scenarios that blur the lines between what's scripted and what's improvisation based on the unassuming public that wander around. At one point I found myself wandering at the far side of the event's space when I heard cries of "fuck off", "get out of here", and so on. I spun around expecting to see a group of young men swaggering up behind me, only to find an elderly couple strolling past, trying to make their way to the nearby bus stop. Frequent references to cameras, and particularly a couple of comments about being "watched" and the need to "avoid their gaze" sent me into a paranoid frenzy, suddenly noticing the large number of smartphones pointing in all directions, snapping pictures, watching, recording.

Whether you think Monument is a successful, or indeed interesting performance, will largely depend on how easily you allow yourself to be sucked into its world. It tries not to show you things directly, and for the most part you'll miss the majority of the "action" that takes place. For me that was fine. The use of the app (appropriately titled Wiretapper) to listen in to conversations and monologues, lends a voyeuristic quality to the show that's only heightened when you catch a glimpse of the characters you're eavesdropping on. That said I can see how the idea of wandering around areas of Central London, listening to conversations on headphones, could be seen as an uninspiring way to spend an hour.

The other key issue is that whilst the performance takes a little while to warm up - it wasn't until 20 minutes in that I really started to see the beauty in its premise - it all ends rather abruptly. It's surely to provide a sense of dropping the audience back in to reality, a sudden stop after an hour of exploring other people's lives and secrets, but it all feels a little anti-climatic. Interestingly, on the night I attended at least, the performance ended in a more secluded spot in London, meaning that as I took my headphones off I easily identified my fellow participants. I believe this was intentional as it allowed us to recognise one another for the first time, encouraging us to discuss what we'd seen and heard excitedly, like friends who'd discovered they each knew the same secret - one they'd carried as a burden.

Monument's not going to be for everyone, but there's something wonderful at the core of it. A simple idea done well that adds an element of danger and intrigue to wandering around London. It also offers an alternative view of familiar locations, making you aware of the stories that happen around you every day and even exploring our own fears and prejudices along the way. As immersive theatre and performance seems to get bigger (such as the work of Punchdrunk and Secret Cinema) Monument strives to make it smaller, more personal, and on the whole succeeds.