The first thing to say is that this performance is superb, Katie Jarvis thinks. But if you’re expecting pure fun - Wild West Stetsons and leather chaps - be prepared for some uncomfortable jockstraps chafing underneath

Great British Life: Ashley Day as Curly and Belinda Lang as Aunt Eller in OKLAHOMA | Photo: Pamela RaithAshley Day as Curly and Belinda Lang as Aunt Eller in OKLAHOMA | Photo: Pamela Raith (Image: Image is copyrighted do not use without permission. www.pamelaraith.com)

The first thing I have to say – the most important thing – is that Oklahoma! at Cheltenham Everyman is amazing! Fantabulous! The singing; the dancing – oh my word, the choreography; the costumes; the set; the wonderful, wonderful cast… No wonder the audience stood up at the end, and clapped as they must have done when George Washington Steele was sworn in as Oklahoma’s first territorial governor back in 1890. Yee-haw!

It’s the story of the wild, Wild West, of course. Of how the state of Oklahoma came into being. Or, sort of. The stage is pretty much devoid of the Native Americans who must have looked on in astonishment as, in a single day in 1889, more than 100,000 would-be settlers and cowboys arrived to stake a claim for land. And even more astonishingly - those Native Americans must have thought - they got it.

So the curtains open to an innocent scene of joshing. The wonderful Aunt Eller (Belinda Lang, you’re an absolute gem) walked across stage with that stiff, gaunt, razor-thin, old-lady gait that somehow sums up Wild West women who’ve lived a long enough life never to see 30 again (possibly older, but it was mighty hard back then), As yeller Eller mangled the washing, we meet the love interest: the gorgeous Ashley Day as Curly, and delightful Charlotte Wakefield, as Laurey. And as the strains of Oh, What a Beautiful Mornin’ hover mesmerizingly over the audience, we’re all truly hooked and lasso-ed. Will they/won’t they? Curly clearly wants to.

So far, Rodgers & Hammerstein, this is The Sound of Music, though with slightly less comfortable jockstraps. (And I have to put my cards on the table, here, because, readers, I have never seen Oklahoma! before. (I was fooled by the exclamation mark into thinking this was purely a sing-along-a-laugh).)

But, no: because from here on in, this musical gets darker, deeper, more interesting by the second. And very much less comfortable.

The change in pace slams into us like a car rear-ending at a junction. And the moment comes when Curly goes to see his (sort of) rival, Jud Fry (Nic Greenshields: I thought you were utterly wonderful).

Great British Life: Gary Wilmot as Ali Hakim in OKLAHOMA | Photo: Pamela RaithGary Wilmot as Ali Hakim in OKLAHOMA | Photo: Pamela Raith (Image: Image is copyrighted do not use without permission. www.pamelaraith.com)

We discover that Jud is passionately obsessed with Laurey; we know he’s but a poor ranch-hand, ill-educated and ill-thought-of (if at all). And when we walk into his hut, alongside Curly, we suddenly discover something else, too: his stack of naked-women photographs. Because, yes, this is the Wild West; and this isn’t just a musical about sexual awakening. This is also about sexual frustration. (Had this been set in Australia or, say, Wales, there would undoubtedly been a dream ballet sequence in which no sheep was safe.) And it gets worse: jocularly throwing a rope around a beam, Curly begins to persuade Jud of the advantages of suicide by hanging: he really ought to give it a go! We’re wooed by the song Pore Jud is Daid – what a sublime blend of voices between these two performers – before we quite get that we’ve just seen bullying, pornography and extreme harassment. (If this were The Sound of Music, this would be the moment in My Favourite Things where Julie Andrew’s head unexpectedly starts spinning round 360 degrees.)

And yet, why not. The point, I assume, that Rodgers and Hammerstein were making (and forgive my ignorance of their art, here), is that the Wild West was not a simple, likeable story. As the programme points out, the closest the musical comes to any kind of overt political judgment is in the song The Farmer and the Cowman Should Be Friends. Yet, actually, the whole thing is riven with awkward questions about justice, provocation, poverty, prejudice, human nature, sexual drive.

And it’s funny, too. I probably should say that. The comedy is particularly focused on Ali Hakim (Gary Wilmot - again, what a great performance!), who woos girls spectacularly successfully. To his utter horror.

And, of course, there’s Lucy May Barker as Ado Annie Carnes, the girl who can’t say no. And James O’Connell as Will Parker (he certainly should), her long-suffering beau. And the rest of this well-chosen, energetic, talented cast.

It’s hard to say that this is delightful. Though it is. It really is. But it’s so much more than that. An iron fist in a velvet glove. A golden gun. Rage Against the Machine’s Killing in the Name sung to the tune of Zip-a-dee-doo-dah.

If you love and know it, go and see it. You’ll be wowed by this production. If you don’t know it, book at once. Because you should, like me, get a whole lot more than you bargained for.

Great British Life: The cast of the National tour of OKLAHOMA | Photo: Pamela RaithThe cast of the National tour of OKLAHOMA | Photo: Pamela Raith (Image: Image is copyrighted do not use without permission. www.pamelaraith.com)

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• The Everyman Theatre is at Regent Street, Cheltenham, Gloucestershire GL50 1HQ, box office 01242 572573; www.everymantheatre.org.uk