There is an alarming sense of cogs being wound in this prison drama by Ed Edwards. It is set in a fancy new Manchester nick where conditions are uncommonly good: a 25-day riot in the notoriously crowded Strangeways has fast-tracked the introduction of a softly-softly regime. There is one man to a cell, so tensions should be minimised, and only troublemakers are subject to frequent searches. Yet every time Edwards introduces a new inmate, he takes us one step closer to catastrophe.
It is like a laboratory experiment in which a mad scientist adds unstable chemicals to a test tube and turns on the Bunsen burner. The steadiest element is Frankie Donnelly, a recovering addict who is in for three-and-a-half years for importing drugs disguised as bars of chocolate. On his side is an even temper, a typewriter and a history of political activism; beyond that, he is at the mercy of his fellow inmates.
They include Kenny, a vulnerable and volatile victim of sexual abuse; Robert, a “white Muslim” drug lord; Bron, a former British soldier with a lethal form of PTSD; and Tommy, who may or may not be an IRA operative. Placating one could trigger another. A chain reaction seems inevitable.
Amusingly, Edwards plays against the tension as well as leaning into it. With the audience on tenterhooks, a whimper can be as much fun as a bang. None of these men is precisely the monster you expect. There is humanity, tenderness and care even in this brutal environment.
The playwright could find no better interpreter than Mark Thomas, extending a relationship that began with England & Son, a play that managed to make a link between juvenile offending and colonialism. Strident and demonstrative, he powers his way through the script under the direction of Charlotte Bennett for Paines Plough, ratcheting up the stakes and jumping brilliantly from scouse narco to Belfast terrorist or bent screw to create a distinct and distinctly dangerous set of characters.
Behind the time bomb humour lies a subtle analysis of the social and cultural reasons these damaged men are behind bars. They are not innocent, exactly, but men swept up in forces beyond their control.
• At Summerhall, Edinburgh, until 25 August
• All our Edinburgh festival reviews
