Andrew O'Hagan's 1994 debut The Missing was an arresting, genre-defying work – part speculative memoir, part Orwellian social reportage – that investigated the phenomenon of displaced persons. Now it has been reconfigured, in his own adaptation for the National Theatre Scotland, as a form of a dramatic requiem dedicated to the unknown. Hearing O'Hagan's words spoken aloud induces the kind of shock he must have experienced when a senior police officer dismisses Fred West's victims as "killable", or an overworked administrator explaining that the official abbreviation for such unclosed files is "mispers".

It is the small details that become most poignant, such as the body of an unidentified woman found floating in the Thames with a pair of child's shoes in her pocket; or a grieving father whose son went missing in the Algarve, and obsessively studies a Portuguese dictionary in the hope of holding a conversation on his return.

This interviewee introduces O'Hagan (or more specifically, his younger self played by Joe McFadden) to the Portuguese concept of saudade which has no direct translation in English, but implies the longing that remains once someone has left. The various voices are steeped in saudade, with the roles distributed among a six-piece ensemble irrespective of appearance or gender – at one stage Brian Pettifer jarringly refers to himself as "a woman of my age".

John Tiffany's production, played against the digital transformations of a giant electronic curtain, has an austere, disquieting beauty, and Imogen Knight's slow, deliberate choreography establishes the sense of lost souls locked in limbo. But The Missing has the same, elusive quality as a drama that it did as a book – an elegiac, emotional tug that is hard to put your finger on, though the Portuguese have a word for it.