Dozens of filing cabinets line the edges of Miriam Buether's set for The Land of the Living. Folders, notebooks, envelopes and box-files cram on to the shelves: evidence of years of research and mountains of data on displaced people. But behind every statistic is a person -- and that's what David Lan's play so potently brings home, diving into one (fictitious) story based on real historical circumstance.

Based on interviews with Austrian-British journalist Gitta Sereny, The Land of the Living plunges us into the turbulent, shattered aftermath of the second world war. But as we watch it, we can't help but think of the many children who are being killed, displaced and traumatised today. It's an overlong but deeply humane play, driven by a powerhouse performance from Juliet Stevenson at its centre.

It begins in 1990 with a challenging encounter between Ruth (Stevenson), now in her 60s and living in a small London flat, and Thomas (Tom Wlaschiha), a decade her junior and fired up with anger and hurt. He wants answers about Ruth's intervention in his life 45 years previously. Those answers will be hard and complex, illustrative of the difficulty of doing the right thing.

As she begins to explain, the end of the war spills on to the stage. We learn that Ruth, as an idealistic 20-year-old, worked for the United Nations Relief and Rehabilitation Administration, tasked with trying to return displaced people to their homes. Gradually, as Lan's play shifts back and forth between 1990 and 1945, we learn how Ruth and her colleagues stumbled upon the Nazis' Lebensborn programme, Himmler's horrific plan to kidnap "racially pure" children from eastern European countries and rehouse them with German families. Thomas was one such: a blazing, talented boy, played superbly by Artie Wilkinson-Hunt on opening night. Aghast, Ruth determines to return these children to their rightful parents. But Thomas, in the end, will not be among them. Why?

There are points when we get tangled up in the weeds of the story, as Ruth wrangles with bureaucrats and struggles with the macro-politics of the developing cold war. A couple of sequences -- such as a long one on a train -- could certainly be trimmed. But Lan's play burns with care about the casualties of horror and the dogged determination of those trying to do something good in the face of unspeakable evil.

Director Stephen Daldry is immensely gifted at creating vivid, fluent onstage worlds, and, in his hands, the chaos of 1945 tumbles across the space, with characters flying through doors, scrambling through the audience, bursting up through a trapdoor. An expert ensemble brings individuals to life with just a few brushstrokes -- most notably Kate Duchêne as Ruth's kindly, flustered deputy, Dora. Buether's set ranges the audience, like witnesses, either side of a long, narrow platform, painted with a map of Europe. And at the heart of the piece are three terrific performances: Stevenson, shifting moment-by-moment between her older, troubled self and the young, zealous Ruth; Wlaschiha, bruised and confused as the older Thomas; Wilkinson-Hunt, raging across the set as the traumatised child.

Lan, Daldry and Stevenson have all repeatedly used their voices and their art to address the trauma of displaced people. Here they combine to create an imperfect but gravely topical and very moving work. Perhaps most potent of all is Gareth Fry's sound design, which fills the space with the voices of unseen children.

The Land of the Living review -- Juliet Stevenson's powerhouse performance drives second world war drama