"The night 35 people walked out was probably the best show I'll ever have," says Philip Ralph. "It was at a community centre north of Newcastle and I was following in the successful wake of The Snowqueen.

"Half the audience were well into it but the other half were storming out and looking at me like I was Satan. It was real whites of the eyes stuff, like being in a gladiatorial arena. I didn't come down for two or three days."

Such is the electric, divisive power of Hitting Funny, a new play from the renowned Volcano theatre company in which Ralph plays a stand-up on the ropes. Chris Rich is a comedian specialising, like all comedians these days, in bemused observational humour. He's supposed to be honing his new show, Things I Don't Understand, whose all too familiar subjects are women, sex, work and politics.

But Rich is in difficulty - torn between the need to make people laugh and the desire to say something authentic, he finds you can't observe the world closely without finding something nasty in it.

"The show takes the form of a standup routine," explains Ralph, "and I can't tell you what a thrill it is when people are actually laughing at my jokes. "But then, about halfway through, Rich goes completely off the rails and the material ventures into the absolutely reprehensible. I don't pick on anyone, I don't drag anybody on stage. The embarrassment comes from me saying things in a room full of people that you really shouldn't."

"Whoever you are," Ralph continues, "I'm gonna say something that will offend you. You might not be offended by coprophilia but you're probably gonna be offended by paedophilia, and if you're not offended by paedophilia you'll be offended by racism.

"In terms of the play, I need to get to a place where the audience is just burying its head in its hands going, why are you doing that?' " But Ralph, who harbours a particular hatred for Ben Elton, says he was inspired to write this exploration of free speech by "what has happened to comedy in the last 20 years".

"I absolutely bought into alternative comedy," he says, "and then I watched as every single one of them sold out."

In his attempt to find an authentic, dissenting voice, Chris Rich strays so far beyond the boundaries of taste that Volcano theatre have had to amplify the "adults only" notice on their publicity.

Ralph, meanwhile, is so convincing in the role that many audience members still leave thinking they've seen a comedy show. Often they write to complain "I came to say Chris Rich's show and I was appalled" but for Ralph, a great believer in "lifting up the stone and making you look under it", there's only one way entertainment can offend.

"I was at my Dad's up in Yorkshire," he recalls, "and he had on this Westlife programme. They'd jumped on the Robbie Williams bandwagon and released a Rat Pack album, and in order to promote it, they were doing a kind of pseudo Pop Idol looking for a girl to sing on one of their songs.

"It had no artistic quality, no worth, and all they were trying to do was flog you s***. I was so deeply, morally offended by that I can't tell you."

Starts on May 12 and 13 at 10.30pm, starts Saturday, May 14, at 9pm. Tickets £8.50/£5, call 01273 709709, adults only.