Dennis Kelly's Debris is a masterpiece. The richness of its language, the vividness of its description and its sheer, brilliant peculiarity make it to my mind one of the seminal theatre scripts so far this century. But in performance it's a difficult one to get right and this new attempt from the young company playON achieves something of a mixed success.The play is a series of intricate monologues and darkly comic role-play, shared between a brother and sister, Michael and Michelle, played in this production by Will Hughes and Lily Knight. Their stories are sometimes realistic, sometimes fantastical, often magnificent and always deeply, touchingly, unremittingly human. Michelle's revolve around elaborate explorations of their mother's death; Michael's are about their alcoholic father and the discovery of a small child in the trash; and together they recite their run-in with some child traffickers nicknamed 'Uncle 'Arry' and 'Mr Bought and Smite'.playON's version is very much a play of two halves. Knight is brilliant as Michelle, with just the right level of childish enthusiasm for a good story, but with a dull fear behind her eyes that sets off the darkness of the script behind the most whimsical and far-fetched of narrative directions. She understands both the humour and depth of Kelly's language and how the two go hand in hand. Take for example how she renders Michelle’s caricature of Mr Bought and Smite in their role-play – the levels of the performance are beautiful - the comedy of the impersonation is a seamless, contradictory compliment to the horror of the scenario.Hughes cannot live up to her. His performance of Michael aims at the urgent and frantic, but comes over hasty and bullish. He loses the humour of Kelly's script and spits out intensity from the first words rather than letting it brew and build with the play’s language. It’s not awful, and it’s certainly not phoned-in, but nor does it move the right ways – it doesn’t blow you away. And, with a play like this, it should.It's never dull to see a play like Debris unravel in front of an audience but playON lack the grace, and the occasionally noisy, disruptive space at Zoo Roxy lacks the solitude, to do it full justice. It's worth seeing, but like the lives of its protagonists, it remains a remnant of something that could have been much better.