Look beyond the star rating here, for this is one of those shows that it's hard to judge by the standards of a conventional Fringe offering. For some, the fact that this is quite unlike anything else in the programme will be enough to make it a must-see.

Unopened Letters From My Mother isn’t so much comedy as an affecting performance art experience, in which comic Lewis Schaffer prods away at the scab over a deep emotional wound, just to see what might happen.

It’s the sort of uncomfortable reality not seen since Kim Noble was in town, with his transgressive, uncomfortable work. And, as with Noble, you come away thinking Schaffer is not the best of people.

Born in New York, Schaffer moved to the UK almost two decades ago in pursuit of a woman and career success. Neither worked out. His marriage, to an agent, collapsed – but not before producing a child to tie him to the UK, while his brand of chaos comedy proved unsellable. 

This is a story he’s told many times before, and the opening ten minutes or so of this show is a very funny recap of that history and his current situation – in a wonderful Woody Allenesque line he explains how he oppresses himself. It’s all delivered with his unique combination of supreme arrogance and gnawing self-doubt. ‘I’m Lewis Schaffer, and I’m funny,’ is his opening line, an assertion that’s as much for his benefit as it is ours.

But in moving to the UK he became estranged from his mother, a situation that appears to have arisen purely out of circumstance, rather than any major row, but the upshot was that he wasn't there when she wanted hm to be.

She wrote to her son multiple times before she died alone in New York six years ago – but the comic never opened the letters, save for a few, just to see if they contained money. Earlier this year he discovered the cache of correspondence at his home in South-East London and instantly thought: Edinburgh show! The premise of which is that he’ll open a different letter each day and read for the first time what they contain.

‘It could be really funny, sometimes it’s just horrible,’ he admits. Tonight it’s closer to the latter, as the letter – scrawled on the back of a crossword puzzle – berates Schaffer for not being with her, save for a scant hour’s visit a couple of days before, and contains a desperate plea for $50,000 to ease her predicament.

The letter calls him a ‘shit’ for abandoning her at 73, and sympathies do lie with the woman whose child turned his back on her. Schaffer paints her as a strong, smart woman – some points, at least, for being a good son there – but that only makes him ghosting her all the more awful.

Schaffer is proud that he doesn’t cry during the show – though he comes close – while insisting the previous night’s performance was a smash. But it’s hard to expect him to entertain while going through such turmoil in front of a room full of strangers.

Instead, the rest of the show takes the form of a Q&A session where the audience get him to fill in some of the gaps in the narrative and quiz him about his motives for doing this. Winning an award seems paramount. Though on a technical level, he hasn’t built enough of a show around the dramatic moment - just leaving it to fate what happens.

There’s a lot to unpack in this foolhardy mix of arrogance and artistic bravery. Schaffer clearly has unfinished business with his mum, and this very public facing of the issues doesn’t entirely seem helpful. He doesn’t feel much catharsis, he says, and he admits that without the prospect of a show he would have simply ditched the correspondence unread, because of the horrible pain they cause him to read.

Unopened Letters From My Mother is billed as a comedy show, and sometimes it is darkly funny, such as in Schaffer’s comparison of the UK and US health systems. Mostly, however, the emotions are still too raw for that.  If comedy = tragedy + time, the fact Schaffer is still coming to terms with his personal misery, with a fresh twist to the ordeal each night, finding the funny in this is a big ask. 

But as the improvisers say, every show will be different, and maybe the show –‘fascinating sociological experiment’ he calls it, rather than comedy – will mix big emotion with big laughs on the night you see it, as he promised us happened the night before this one. 

What he gets out of it is harder to ascertain. If it’s the approval of strangers, it backfires, as he’s not judged too kindly on the content of today’s letter. And this probably isn’t the ideal substitute for real therapy.