The BT was packed for Criminal, a comedy show/podcast in which a detective and three suspects have to improvise a murder investigation. And, judging by the number of people who'd seen the show before, it's gaining in popularity. In the anonymous black space, and armed only with three chairs, the improv format gave us all the fun of radio where one scenario turns into another, with the addition of watching the performers mime their way out of a hall of mirrors and into a sauna.

Criminal is the brainchild of Joel Gatehouse and Ralph Jones, who host. Under their guidance we learn that poor Caroline Vaughn is dead. But what was found on her body? Where was she discovered? And what profession had she practiced when alive? Inevitably we got the immediate suggestion 'hooker' for her profession. The hosts did their best to modify this to a rugby position, and the suggestion was hardly their fault. But the fact it got thoroughly incorporated into the storyline (despite the official decision she was in fact a chimney sweep) was a bit disappointing.

They've given themselves a tough job with the set-up - the hosts take suggestions initially, but there's no further audience participation, and no omniscient narrator to steer the plot, inject a new idea, or give the detective breathing space. The three suspects are given cards to determine which one is guilty, and the detective must guess whodunnit as well as making a coherent story! Of course they're playing it for laughs, but they're not sticking to a well-known trope like an Agatha Christie country house set-up, and I get the impression they're going for something a bit grittier and dramatic.

Our detective was played by Alexander Fox, who performed a show about Ringo Starr at last year's Offbeat. Joel Gatehouse transformed himself from host to suspect, alongside Sally Hodgkiss and Frankie Evans, the latter a veteran of the Oxford Imps. Sally was my favourite actor, secure of accent and solidly inhabiting her various characters. As the detective's wife, her gruff catchphrase 'What have you brung me?' was brilliant. Joel handily played all the other incidental characters, including the Irish Sea.

There were some moments of pure random brilliance, when people reached for a simile and their minds rebelled. 'I'm so weak, my arms are like... pebbles,' declared Frankie. 'Oh the shame. What will people be saying in the... the... the catteries?' demanded Joel. And I loved the unexpected scene where our hero goes to interview the rugby coach, finds he's also a life-drawing model, and comes out with some answers but also a portrait.

Not every scene took off like that, and I suspect I wouldn't see eye to eye with the philosophies of all the comedians. (Of course the downside of being unscripted is that sometimes under pressure you reveal more of your own attitudes than you meant to.) But a whole show as good as the best bits would be asking a bit much of any group of improvisers who don't necessarily know each other. But I would definitely go and see Criminal again. And fortunately there are some podcasts available on their website, so you can check out the format, and catch up with a whole host of other investigations into some very funny goings on.