Some reviews I have seen of the Old Vic’s superb production of Duncan Macmillan’s two-hander Lungs have made much of the evident rapport, the sexual chemistry, between its two stars, Claire Foy and Matt Smith.

This arises – it is alleged – from their having worked together, in great amity, playing The Queen and Prince Philip in the popular Netflix series, The Crown.

Frankly, this is baloney – and moreover an insult to two fine performers. Foy – an alumna of the Oxford School of Drama – and Smith, a former Dr Who, are consummate professionals at the peak of their powers.

There would be rapport, and all the required intimacy, in their portrayal of a couple so very different from the royals – the most senior ones, at any rate – even if this were their first time on stage together, and hated each other’s guts. It’s what actors do.

But perhaps not quite so well as this starry pair, who have made Macmillan’s 2011 play, directed by the Old Vic’s boss Matthew Warchus, the must-see theatre event of the moment.

The subject of the piece is parenthood, which is explored during 90 interval-free minutes to the exclusion of nearly everything else. Such as we learn of the couple – which never comes to include their names – is gathered steadily during their discourses on baby, if baby there is to be.

A persistent, indeed principal, theme is the agonising matter (for some) of whether it is a good thing to bring children into the world, given their heavy carbon footprint. (Certain royals come to mind . . .)

From the prospective mum – rarely other than glum, and always earnestly right-on in her pronouncements – comes the claim that junior will emit the weight of the Eiffel Tower in CO2 during her/his lifetime.

Whether this is true, we can’t be sure, although it does eventually emerge – despite her hard-to-curb smoking – that she is a doctor. Her partner is an easy-going – well, certainly easier than her – musician, who comes to ascend the corporate ladder in a business never specified.

That we are in the land of the middle-class is evident from the play’s start, when the bloke proposes parenthood during a shopping trip to Ikea. Cue an explosive response.

“Where do we go from here?” he asks, as the row goes on. “Well, we should try to leave the car park.”

Yes, there is welcome humour. sometimes mordant, during action that whips us through time and different places, though never without clarity as to location.

We could in fact be anywhere, thanks to the versatility of Rob Howell’s simple set – with two coffin-shaped plinths in transparent plastic on a stage of solar panels – lit from above (Tim Lutkin) by a profusion of stark white beams.

Though the play is short, its single-subject navel gazing can be trying. But it is compelling, not-to-be missed theatre.

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