Two muddy-kneed boys, their shirts covered in badges, roll around happily on the floor. They’re telling tales of lakes and rope swings, of pride in the flag and the rules for being men – all while playing the Beatles’ Ob-La-Di, Ob-La-Da on matching harmonicas. Their nicknames are Ace and Grasshopper, they’re American Boy Scouts, and life will indeed go on for them… out of President Lyndon B Johnson’s 1960’s, and into a bruising future.

The boys are actually played by two women, Xhloe Rice and Natasha Roland, and the clowning physicality of that opening scene is very much the essence of their show. A truck tyre – itself a symbol of rugged masculinity – is the only piece of set, but they find a seemingly endless array of ways to use it: as a platform, as a hide-out, and most poignantly as a way to avoid looking at each other when there’s something painful to be said. They pretend to be soldiers, stay up after lights-out, and launch a daring night-time raid on the grown-ups’ stores.

It’s good wholesome fun, a blend of sweet vulnerability and perfectly-captured boyish bravado. But the shadow of something darker is rarely far away, and – in a quietly horrifying scene set after a baseball game – we soon come to learn what society (or at least their dads) think true manhood means. Flash-forwards to the future begin to intrude. We’re watching the boys lose their innocence, and it’s all the sadder because they’ll only know it happened much later on.

And perhaps the same can be said of a nation. To Ace and Grasshopper, the LBJ of the title is a man to be admired and revered, whose very name conjures almost mystical power. It’s hard to imagine any modern-day president being seen in quite the same light – and as the boys grow to men, to find their childhood adventures echoed in quite a different context, there’s a hint that we might be witnessing the reason why.

It’s a complex, many-layered story which asks a lot from its audience, and for me it does pack in one extended allegory too many. But the characterisation is faultless, the synchronisation is perfect, and the physicality of the performance is mesmerising to see. Witty at times and moving at others, it’s a study in friendship and the charm of youth. And the wistful regret – for a time and a generation lost – lingers long after the truck tyre’s been packed away.