With this show, part-cabaret, part-concert, part-play and part-sketch comedy, The Suitcase Royale define, or defy, what constitutes theatre.

For the Sydney Theatre Company, by design or happenstance, the timing is interesting, since it coincides with the inaugural, long-overdue Sydney Fringe Festival. While I’d be one of the first to laud and applaud it, the fact is at least some of the shows under its auspices haven’t really been what one might typically associate with fringe. Indeed, some has been downright mainstream, if not mainstage. The irony is that STC should bring one of the edgiest, fringiest show to Sydney. I’m not sure what it all means, but it is piquant. And another feather in the cap of Cateandrew.

When we walk into Wharf 2, TSR, as ‘rag ‘n’ bone’ garage band, are belting out their Tom Waits swamp-blues replicas. They cut a retro-chic figure, or figures; shadowy, gravelly, gruff, grungy and impassioned. But they don’t hail from the bayou, or anywhere remotely near the Mississippi. The soupy Yarra’s more like it. They’ve been gigging together for half-a-dozen years or so, first making a name and carving a niche with Felix Listens To The World, which premiered at the 2005 Melbourne International Arts Festival. On the strength of it, they toured at home and across North America, garnering great reviews wherever they went. They’re about to cop another one, too.

While belting out their woeful ballads, they are shrouded by sad plants and grasses, dehydrated and drooping. Casting one’s eyes to the left reveals the Grimm detail of their junkyard set, a richly imaginative, resourceful and inventive catastrophe of detritus which wreaks a dark, magical storybook spell. There’s a miniature, illuminated hotel and abbatoir-cum-boxing gym, where we first meet Backbone Joe; his eloquent pugilistic motto: “I never lose!”

While it’s hard to imagine a boxer being as thick as two planks, we are inveigled to suspend disbelief and invest in Joe’s substantial IQ deficit. His exploitative mentor, Messy Dimes Dan. “You’re like a son to me, Joe,” confides Dan, tenderly. “You’re like a son to me, too, Dan,” rejoins the idiot sans savant. It’s a line that’s emblematic of their wacky, bent sense of ridiculous, absurdist humour.

When Dan parks his ‘car’, comprised of a modified Globite suitcase, by folding it up, he explains: “It’s a convertible.” As an aside, Dan says, confidentially, to the audience, “that’s my favourite line in the whole show”. There’s plenty more ad-libbing where that came from, much of it dangerous free association, which makes a very funny script even funnier and, paradoxically perhaps, seeing the performers putting themselves right out on the edge, especially since it’s effected so confidently, seems to draw the audience even closer.

Joe hones his skills the hard way, punching-out sides of hung meat, bareknuckled, in Dan’s gymattoir, oblivious to Dan having done away with his missus. That’s what the cherry-flavoured horse sedatives are for: Dan’s convinced Joe he needs them for his blackouts, of which they are, in fact, the cause. When Joe is alerted to Dan’s deceit, he’s incredulous. “Why would he give me cherry-coloured horse sedatives?!” The answer is, of course, as plain as the noses on our faces: “They ran out of orange.”

Characterisations are richly comical, as are Miles O’Neill, Joseph O’Farrell and Glen Walton’s unexpected stage directions, which side-splittingly kill off any propensity to be suckered into a dramatic moment.

Book into a cosmetic surgeon apres show, with a view to silicone implants, because you will laugh your tits off.

The details: The Ballad of Backbone Joe plays Wharf 2 until October 2. Tickets on the STC website.