Rehearsals for The Bear Pit
Rehearsals for The Bear Pit
© The Other Richard

Coupledom feels like a catastrophe in The Bearpit. Two millennials are marking their fifth anniversary – "the big oh-five" – with a night out in an apparently apocalyptic London. They pop into a pub just as a mass brawl erupts. They skip past riots in Soho and hop over bodies to get into good restaurants. London is cocooned, cut off from the rest of the country. Oceans levels have risen and the sun is bearing in. Dead whales lie rotting on British beaches.

It's like a tourist trip through a feral metropolis; a date night as the modern world falls apart. Their cheery tone belies the unfolding anarchy, as knife fights erupt and the Thames breaks its banks. What's left beautifully ambiguous is whether love shields them from the chaos or blinds them to it. "It's not our fault we were born at the beginning of the end," they shrug. Millennials gonna millennial. Post-crash, post-austerity, post-Brexit, what other hope do they have?

In the midst of all this, Nic McQuillan and Flora Marston cling to each other – though whether out of love or out of fear isn't ever quite clear. They hold hands insistently, then grasp each other's wrists as they lean out. Are they keeping each other safe or captive? Holding on or holding back? Their words betray an anxiety about becoming one; two lives coming together, two bodies coalescing. It's like they're joined at the hip – and it's increasingly apparent that they really shouldn't be. He's insufferably overbearing, and she can't escape – or rather, she won't.

Why would she when the world beyond them holds such terror? Or is that just a story he's spinning? A tale told to entrap? Brilliantly, The Bearpit allows it to be both, real and not real at the same time, and the play hooks the slow drift into marriage to our sleepwalk into climatological collapse.

The Fringe has thrown up some thrilling new voices in the last few years. Lulu Rascka's Nothing and Sam Steiner's Lemons, Lemons, Lemons, Lemons, Lemons, both debut plays, fused a fresh sensibility with a performative kick. Like them, The Bearpit makes the most of what little it has: a bare stage and some bodies. It is brilliantly (and unusually) co-written. McQuillan and Marston combine a whiplash cruelty with a crafty sense of humour (the sun is so close it photobombs selfies), and their text rattles off like a runaway train, then slams into plainspoken, heartfelt simplicity. "You suffocate me," she pleads. "I want to devour you," he replies. The violence is barbed; the hearts are bleeding. McQuillan and Marston aren't always in control – The Bearpit takes on too much at once – but their writing's like a firework going off in your face.

It's amplified by two electric performances – McQuillan's smile gives way to a sneer and Marston practically tears herself in two – but also by a series of short, sharp little interludes. A pop quiz on capital cities turns into torture, and press ups stand in for one-sided sex. Physically, they wrestle and balance and tread on each other's toes. A game of slapsies gets out of control. It's exhilarating, tense as a tightrope, sharp as a needle, but it's artful too. At times, their bodies recall Marina Abramovic works; her and Ulay holding a bow and arrow between them.

That piece, Rest Energy, is one of the most articulate expressions of romantic relationships ever made. It catches the way trust and threat go hand in hand. The Bearpit says something similar – the thing that keeps you safe may be the biggest danger you face.

The Bearpit runs at Zoo Southside until 19 August.

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