In his programme biography, neophyte author Stephen Jackson states that he makes ends meet by also working as an inventory clerk. If there's any justice, he'll leave the office world for good and its loss will become theatre's gain, at least if this lunatically brilliant little musical comedy is anything to go by. Set in a Brummie diner - possessed of that peculiar brand of kitschy depression that can happen when hopeless British suburbia meets brash Americana - this has the makings of a cult hit along the lines of Rocky Horror or Hedwig. It is utterly bonkers but also screamingly funny.

The basic set-up - gormless burger flipper PJ (Ricky Oakley) goes out with the boss's feisty daughter (Lucie Shorthouse) while long-serving waitress Jean is secretly in love with the boss himself - is blown apart by the arrival of mysterious Polish (or is she?) "bad immigrant" Marika. She's a woman with a secret, a past and one hell of an attitude, who offers to work initially for almost-free, being paid only in diner food. On top of all that, Jackson hurls in jealousy, gang warfare, prostitution, intergalactic space travel, possible demonic possession, the immigration authorities and a failed package holiday to occasionally bewildering but consistently entertaining effect. After the initial couple of moments almost nothing in the plot goes in the direction you might expect.

There's also a lot of music, albeit few full numbers, and the soft-rock score is pretty catchy, although there is every chance you may be laughing too hard to notice. Jackson even has some fairly pithy points to make about the attitude of huge swathes of Brits to foreigners. "I like Polish people," says Jean to Marika at one point, "in Poland. But we've just got too many here... still, you'll all be gone soon."

In tying serious points to some truly delightful flights of comic fancy (for instance, diner owner Eddie and his best mate's conversations consist entirely of yelling the names of car parts and DIY tools at each other, but always ending with the word "t*ts!"), Jackson sometimes overreaches himself. That's particularly true in the second half, but he nevertheless announces himself as a blazing new talent: the majority of the material here truly sparkles.

It helps that Soho artistic director Steve Marmion has given Roller Diner the best of all possible productions: unerringly knowing exactly how much tongue to leave in cheek, but also allowing the more sinister elements of the piece full rein. Anthony Lamble's set is queasily colourful and the actor-musician cast are uniformly superb. Nobody pushes too hard for a laugh, which shows a (well placed) faith in the wacky material. Joe Dixon gives the diner proprietor a grizzled charm and a touching, unexpected sensitivity that makes it clear what the torch-carrying Jean (Rina Fatania, one of the funniest women currently working on the London stage) sees in him.

As the young mismatched couple, Oakley and Shorthouse are as lovable as they are talented. Shorthouse's gloriously bad tempered waitress becomes ever more jealous of newly arrived Marika to wonderful comic effect, while Oakley imbues PJ with a sweetness that proves genuinely affecting. Plus he has a terrific rock tenor (when they start looking for a London lead for Dear Evan Hansen, they should definitely look in Oakley's direction). David Thaxton is great fun as Eddie's sidekick, and contributes some gorgeous vocals.

Marika herself is a fascinating creation: almost chillingly self-possessed yet occasionally pitiful, hard as nails but with a genuine undercurrent of warmth, she is magnetic but unknowable and she is also absolutely hilarious. Lucy McCormick inhabits the role with a deadpan ferocity that is as unnerving as it is riveting to watch, and she sings like a true diva. It's a stunning performance.

If Roller Diner isn't quite at the sublime level of Our Ladies Of Perpetual Succour - the show it most closely resembles in terms of outrageous comic invention shot through with moments of surprising seriousness and a wealth of good music - it isn't far off, and marks the arrival of a highly original new theatre writer. Soho Theatre should have a massive summer hit on its hands.

Roller Diner runs at Soho Theatre until 24 June.