It's seven o'clock in the evening and there's a crowd forming outside Cordy House in Shoreditch. Enormous inflatable tentacles reach out across the street from the building's roof. Three people are hard at work in the loading bay, building a Rube Goldberg water feature. Their hard hats occasionally spray the queue with water.
No one in the crowd seems sure of what's going on. Most have turned up on the recommendations of friends, with little or no idea of what awaits. Even the door staff are confused by the ridiculous phrase, "I'm here for You Me Bum Bum Train".
On some levels this is strange, because You Me Bum Bum Train has been running in one form or another for around eight years. Part theatre piece, part installation and part rollercoaster, it began life in Brighton and returns every so often like Brigadoon.
Once past security the source of the confusion becomes apparent: Cordy House is also home to the Mutate Britain exhibition, which accounts for the majority of the crowd. You Me Bum Bum Train occupies a tiny booth in the corner of the gallery.
Beyond the sign-in booth is the world's smallest karaoke bar. Free rum cocktails are available - and necessary to loosen up the inhibitions, both for the karaoke and the Bum Bum Train itself.
The Train is a labyrinth of rooms, scenes and situations through which the participants ("audience" is definitely the wrong word) pass one at a time, wandering, crawling or being pushed in wheelchairs.
Each room is joltingly incongruous with the last. Emerge from the trapdoor at the end of the ice tunnel and find yourself in a boxing ring; push your way through the nightclub queue to find yourself in a frail old lady's bedroom.
The seventy or eighty performers populating the maze don't allow anyone to loiter. Sometimes this is a relief; sometimes it feels a shame not to linger longer, continuing conversations with some of the Train's more colourful characters.
There's no overarching theme or narrative. The emphasis is on a personal experience: of flying blind into the unknown, of being an active participant yet still a helpless observer, and of a seemingly exclusive artistic underground.
There's a sense of élitism that comes from simply having found out about the event, and that feeling of having discovered something exclusive is part of the attraction.
But there's something problematic about it as well. It isn't only the uninformed who are excluded: narrow crawlspaces and steep stairs place certain physical restrictions on who can and cannot take part.
Additionally, the conclusion of the Train - in which participants are ejected without ceremony or aftercare back into the Mutate Britain exhibition - rapidly dissolves that sense of belonging.
The experience is designed to be fleeting. Many of the scenes in the maze feel cut short before you can fully sink into the situation, and the Train as a whole feels unjustly brief considering the £15 entry fee. Realising that this is an intentional part of the mystique does nothing to dilute the sense of disappointment.
Yet, having said that, the first thing I did after leaving was recommend the experience to several local friends, if only to have someone else with whom to discuss it. The creators can apparently manufacture the marketing Holy Grail, positive word of mouth - an impressive achievement above and beyond the event itself.
You Me Bum Bum Train is not something everyone can enjoy. But whether it's a cynical comment on exclusivism or an encouragement to theatregoers to abandon their role as passive observers, it's evidence of experimentation in British theatre - something the industry can't do without.