Sam Simmons, Soho Theatre

The award-winning Australian comedian Sam Simmons is shuffling around in a pair of bread loaves. He's wearing them like slippers and trying to take bites out of them at the same time. Indeed, his tremendously silly show, Fail, is essentially a shambles. This is the overarching joke: it's his absurd non-sequiturs and his tongue-in-cheek, shamateur performance style that reduce his audience to spasms of laughter.

The persona adopted by Simmons here is that of a fortysomething loser-going-on-raving-loony. Bobbing around maniacally, belly sticking out, he sports a flat cap on his balding pate, a bushy moustache and retro-geek glasses.

We vaguely glean the guy has been ditched by his girlfriend and is on the skids. At the Soho Theatre, he appears to be curiously shacked up, like a squatter, in an office block among desks and PCs which he has littered with cheese graters, a cabbage and some Frosties (though Simmons has, in fact, just plonked his props on the set of Mongrel Island, playing in rep on the same stage).

Anyway, Simmons is hardly bothering to give us a coherent backstory. Half the time, he is surreally trapped in a kind of split-personality quiz show. Slinging himself into a chair, he tries to answer questions that are fired off by a disembodied, amplified voice – his own. It's nutty stuff: "Fiona has lost her egg timer. How can she tell how fast the eggs are?"… "Question number nine: true or false?"… "In which town was Tony Blair built?"

Also randomly, Simmons will abandon the quiz and leap out of his chair. He might grab the cabbage, dubbing it a satanic lettuce, or maybe stick toy eyes on it and sing about what a nice, nice cabbage she is. At other points, he'll start a food fight, tossing sliced bread into the audience. Or he'll randomly launch into preposterous disco-dancing, stripping down to shiny gold underpants – cod-stud one minute, camp the next.

Some of Fail isn't all that funny. Slack patches certainly occur. The show's title, to a degree, lets Simmons off the hook in that respect. More often than not, he contrives to make a rather winning joke out of how this or that bit has bombed – having a laugh about it. And the craziness becomes irresistibly, incrementally hilarious.

For all Simmons's idiosyncrasies, he can seem like a mishmash of other star comedians: a trace of Harry Hill; a touch of Tim Vine; a dash of Sasha Baron Cohen; and maybe just a tinge of Rolf Harris. Don't expect any sharp aperçus either. Indeed, Fail is such silly nonsense that, at points, you might think you're merely watching a kids' entertainer who has gone cuckoo.

However, provocatively unPC gags surface. Ultimately, watching someone being so idiotic and uninhibited is exhilarating, even as a slight edginess creeps in too. Most interestingly, you're left wondering quite where the borderline lies between character comedy and Simmons himself. He goes by his own name in Fail and you sense a vein of autobiography about the midlife crisis.

• Book tickets for Sam Simmons's Fail at Soho Theatre, London W1 until 30 July
• Book tickets for Simmons's new show, Meanwhile, at the Edinburgh Fringe 3-28 August