People Show, featuring 1966 founder member Mark Long - who hasn't slowed down, "aged" or put on one ounce of fat - is the oldest surviving fringe group. They came out of art school, not drama school, and embraced jazz, modern painting and film noir when everyone else was changing the world or reading Stanislavsky.

So it's odd to see this fizzing spoof murder mystery as evidence that everyone else has caught up with them, especially fringe phenomena such as Told by an Idiot and Spymonkey or even mainstream shows like One Man, Two Guvnors and The 39 Steps.

I haven't seen a People Show for 20 years, but The Detective Show has all the old hallmarks of physical elasticity, smoky jazz music (composed by George Khan), and broadminded cultural references; no other fringe group would lace an "impromptu" performance riff on the unsolved mystery of Agatha Christie's disappearance in 1926 with meaningful name checks on The Seven Samurai, The Seventh Seal, Hedy Lamarr, Bob Dylan and the Isokon flats in Hampstead (where Christie lived during the war).

Devised by the three onstage goons - Fiona Creese, Gareth Brierley and Mark Long - with input from Sadie Cook, direction by Jessica Worrall and lighting (as usual) by Chahine Yavroyan - the investigation comprises a trail of red herrings, a missing body, an undercover agent who's also a chief suspect, a tour guide, "an actor" and a spooky seagull salad served in a dodgy restaurant.

The action doesn't so much proceed as accumulate around a series of fast-moving sketches, Jewish gags and wise-cracks (most of these emanating from Long's conflicted comedy persona) and misleading conclusions, so that by the end we don't really know where we are, any more than do the performers, or indeed who we are. It all amounts to an exhilarating, hilarious and unusual experience.

I was dreading People Show hitting me once more as arthritic or old hat; the absolute opposite is the case, and having tuned out previously around People Show 75, I fully expect to enjoy People Show 175 in a few years' time. As Sam Shepard once said, and it's still palpably true, "Theatre without People Show would be like music without rock and roll."