This is the story of one day in the trenches of World War I. Playwright RC Sherriff, who became a Hollywood screenwriter on films such as Goodbye Mr Chips and The Dambusters, fought at Passchendaele, where he said he met all his characters.

Told with flashes of humour, the story is, inevitably, laden with the doom of what’s to come. The initial gung-ho approach to what was seen as a great adventure gives way to a loss of that innocence, and the realisation that they will not all make it home.

The sandbags piled above the stage look oddly in place among the wooden pillars of the Watermill. Lanterns hung from the slips add to the feeling of being in a rather grand dugout; the spare stage hints at the claustrophobia of such a tiny place of shelter. The young Raleigh (James Mack) arrives fresh from school, ready for that adventure, and having asked to be with his old pal Stanhope (William Postlethwaite). But Stanhope’s experience has changed him; whatever innocence he had has long gone, to be replaced by cynicism and barely-contained anger.

When a hopeless raid is ordered, Raleigh and his comrade Hibbert are sent, with terrible consequences.

It’s gripping and beautifully done, even though you know it’s not going to end well. The language of public school – the ‘Goshs’ and ‘I says’ – should be funny, but it simply underlines how unprepared these civilians were for war. And even sadder is the knowledge, through our 21st century eyes, that similar wars are still being fought.