At first, you might mistake Happy for a typical parody Fringe show.  The familiar comic building-blocks are there: the script isn't quite finished, "artistic director" Thom seems slightly unhinged, and one of the actors is missing – leaving Thom's long-suffering friend Carrie to play the part of a gay man.  Sure enough, the first few minutes are very funny, thanks largely to Thom's utter lack of self-awareness and habit of undermining his cast.  But before you settle in for an hour of comfortable humour, beware.  Happy is not a happy show.

There's something much darker lurking in the background – a looming shadow which gradually envelops the play.  Thom isn't new to this business; he's already penned a cult hit called Snakes!, a musical re-interpretation of (yes, you guessed it) Snakes On A Plane.  His creation brought him fame, friends, and excitement… but one day the curtain fell, and the good times abruptly ended.

The show we're seeing now is about what happened next; about the desolate pattern of drugs and casual sex which Thom has slipped into.  It's a world which Thom professes to celebrate, describing shared drug-taking as a "tender moment" and dancing his way through a soulless search on Grindr.  But there's a grim authenticity to all this too – a perfectly-judged air of confession, searing yet believable.  Incongruous musical numbers, sung by Carrie, capture the endless cycle of hope and disappointment which Thom has found himself caught in.

Actor Thom Sellwood (both actor and character are called Thom) runs the gamut of emotions, and is equally compelling in all of them.  He's hyper to begin with but broken by the end, and his erratic behaviour creates increasing tension with a well-drawn, fundamentally likeable persona.  Carrie Marx hits some perfect notes too – at first brightly covering for Thom's flakiness, but later, as his insensitivity towards her becomes increasingly domineering, unable to conceal her disgust.  The only downside to this finely-worked interplay is that it distracts a little attention from Carrie's songs, which are beautifully performed and often surprisingly tender.

And here's the thing that makes Happy uniquely disquieting: a lot of what we're hearing is true.  There really was a cult musical called Snakes!, Sellwood really was the star of that show, and there really must have been a huge void in his life when its three-year run finally ended.  Of course, he isn't really snorting cocaine in the wings and he isn't really having a nervous breakdown on stage – but when the lines between fact and fiction are as blurred as they are here, it's hard to silence the voice in your mind which asks if he just possibly might be.  I'd have enjoyed Happy rather more if it had been more grotesque, offered more reassurance that the extremes we witness are the stuff of theatre alone.

But then it would be a different show.  Happy scores five stars because it doesn't compromise; because it weaves a single jet-black thread, then follows it into the darkness wherever it leads.  At one point Thom pauses the action for a lengthy verbal essay, a convincing analysis of how addiction can consume a life.  Theatrical lectures normally annoy me... but this time I listened, because this time, I felt the privilege was earned.