In the middle of the Second World War, an egg is rescued from a bomb blast; the smallest of miracles, maybe, in the middle of carnage. It's carried to safety in a bloodied hand and incubated in a shoe filled with pillow feathers. Life, in Rita Kalnejais' superb and surprising play, is as delicate as it is hardy.

Towards the end of the war, two teenagers spend the night together in an abandoned house in Chartres, outside Paris. Otto is a German soldier, gawky and sweet. Elodie's a local girl with a full-beam smile. They seem impossibly young: wary of sex, but excited by the prospect; easily distracted by pillow fights and small talk. It's both the most natural thing in the world, and the most abnormal; something so human against a backdrop of barbarity.

As in Robert Holman's plays, a fleeting encounter sheds light on the whole world in This Beautiful Future. Otto and Elodie overturn history. They remind us that war doesn't override love, that ordinary things happen in extraordinary times, fresh hope in adversity. People still feed cats, even as firing squads take aim. Teenagers still kiss. Chicks still hatch. History's too simplistic by half. Bradley Hall's fumbling, fresh-faced Nazi, his grey uniform baggy and creased, disrupts the stereotype of oil-slick SS men, while Hannah Millward's Elodie is all springy life; a light breeze in times we imagine as airless. Behind them, designer Cécile Trémolières puts a sepia image of a stock French village: thatch houses and fields and old, rural idyll.

Both teens are optimistic. Each believes, initially, that the war is theirs to be won, and that their side is right. Whether indoctrinated or not, Otto's adamant that "Mr. Hitler" is doing good; that racial cleansing and Nazi rule are humanity's best hope. He's pushing ahead for his own beautiful future, just as Elodie is hers. Just as we are ours.

Cleverly, Kalnejais' script pushes us to zoom out. Two older actors, Alwyne Taylor and Paul Haley, interject here and there, looking back on their own lives and expressing regrets. "If I could do it again," they'll say, "I'd have done this or that." Jay Miller encases them in karaoke booths, floating war songs and pop anthems across the main action – a rich tonal juxtaposition that fuses the past and the present together, each as modern, as full of feeling and as uncertain as the other.

The future is improvised; history sticks to the script. Not only do Kalnejais and Miller remind us that the past is far more complex than we perceive it, they also warn that our messy present will, one day, be seen as simplistically – judged from afar, in hindsight. For all our certainty now, for all our optimism, some of us will be on the wrong side of history.

This Beautiful Future recognises, however, that that isn't a fixed thing. History is written by the victors, after all, and, as the war ends, it's bracing how quickly we seek redress. Both Otto and Elodie are, in different ways, erased almost immediately: a process as inhuman and arbitrary as anything that came before. Proof, were it needed, that we'll do anything to ensure the future is blemish-free.

That's not the same as beautiful, though, being manufactured and forced. Instead, Kalnejais lands us with a challenge. She insists that we are that beautiful future, as improbable as the egg that survived the bomb blast, and, as such, we owe a better future – a more beautiful future – to those that came before and those yet to come.

This Beautiful Future runs at the Yard Theatre until 20 May.