It’s the final piece of dialogue that defines Our Boys. Joe, a soldier injured in the IRA bomb attack on Horse Guards Parade, on his knees, centre stage, for only the second time allows his impenetrable demeanour to crack and bleeds the pain a soldier feels inside. It’s utterly engrossing.

Cian Barry (Keith), Laurence Fox (Joe) and Matthew Lewis (Mick) in Our Boys at the Duchess Theatre, London Photo: Tristram Kenton

Laurence Fox delivers the dialogue. For the past two or so hours he has kept Joe’s pain locked up as he completes his first year in a ward in a military hospital in Woolwich. It’s a quiet, reserved performance that still manages to hint at the psychological damage inside and the violence to come.

It’s only in these final moments that the play reveals what it is. Until then, the writing and characters are engrossing, while the plot is allowed to build slowly. It is expertly done.

Joe is joined by Keith, a Northern Irish soldier whose numb leg may be psychological and Ian, a 20-year-old who was shot in the head in Belfast. Potential officer Menzies joins them with an ingrowing hair in his bottom and creates a tense class divide.

They are visited by the boisterous Parry and Mick - toe amputation and circumcision, respectively - and the play seems to consist of typical squaddie banter until a party for Ian’s 21st goes wrong.

Cian Barry’s Keith seems to embody it all - a tough talking Proddy who still phones his mum. Barry’s is a warm performance that invites the audience in to the otherwise closed ranks.

This is an ensemble piece, although Fox and Barry are allowed to dominate. As is Arthur Darvill, fresh from dying on Doctor Who. His Parry is the dark half of Joe - also quietly manipulating things, but to his own ends, while maintaining a cheeky demeanour at odds with Joe’s reserved nature.

This British One Flew Over the Cuckoo’s Nest is a deep and powerful drama that is already on the way to becoming a classic.