Lin Coghlan's play pulls off a great surprise. It starts as a grim, post-apocalypse nightmare. But just when you think Coghlan has landed herself with a bankrupt form - does anyone remember the National's Thee and Me? - you realise she is writing about the survival of goodness in a disordered universe.
Coghlan's youthful characters are fleeing a chemical-weapons attack on London, and their own deprived lives. Mac and Deccy have fled a bleak, barren estate. Mac's younger brother, Cookie, is a parkside perv with a religious bent. The militaristic Terry has abducted 15-year-old Jean from a residential home. Even the one older character, Rory, is a screwed-up social worker who scores drugs from his clients.
Clearly Coghlan is using the device of a chemical attack to suggest that many people already inhabit an urban hell. As the play unfolds, we learn more and more about an east London world, full of petty crime and fostered kids, in which anarchy apparently rules. But what lifts the play above sociology is Coghlan's passionate belief in human decency. As the floodwaters rise, Cookie instinctively rescues a woman impaled on a pole, Jean lovingly mothers an abandoned baby and even the misanthropic Rory forms a strange attachment to a chicken.
You could argue that Coghlan goes to extreme lengths - the devastation of London - to demonstrate inextinguishable compassion. Her symbolism is also sometimes obscure: I never quite got the point of the bag of dead monkeys the kids discover in a derelict barn. But Coghlan's play belongs in the tradition of Bond's Saved and Kane's Blasted in its hint of spiritual affirmation in the midst of total desolation. There is even a suggestion of the hovel scene in Lear as the characters huddle together for warmth.
Paul Miller's production, ingeniously designed by Hayden Griffin, also overcomes the inital barrage of four-letter words to invidualise the characters. In particular, Danny Worters's prayerful Cookie, Peter Sullivan's disintegrating social worker and Samantha King's maternal Jean. The quality may be a bit strained but, in pessimistic times, Mercy offers a refreshingly positive moral vision.
· Until August 7. Box office: 020-7478 0100.
