Before the dawn comes up, on a hill overlooking Shropshire in the Welsh borders, an American academic tells a British doctor that “the vertical hour” is that moment in combat, after a disaster, after a shooting, “when you can actually be of some use.” What are the practical responsibilities of medicine? What, indeed, is the best treatment for political wounds?

Nadia Blye (Indira Varma) is visiting Oliver Lucas (Anton Lesser) on a break from Yale. She carries with her the impact of her experience as a war correspondent in the Balkans and a mixed bag of feelings about the war in Iraq which, in the initial stages, she has supported. She has travelled to Britain with Oliver’s son, Philip (Tom Riley), a physical therapist who is scarred in a different way, by his family background.

David Hare’s play was premiered on Broadway in late 2006 with Bill Nighy and Julianne Moore in a production by Sam Mendes. Reports were mixed. Jeremy Herrin’s admirable new staging in Sloane Square is a triumph of clarity, sensitive, well-judged acting and supple argument propounded with intelligence and passion. If the occasion doesn’t achieve full theatrical combustion it is because the actors are projecting more of the play than themselves. The equation may not yet be quite perfect.

But this is, undoubtedly, one of Hare’s best plays, a mature blend of the personal and the political. Almost every line makes you think. The playwright allows less time than usual for his trademark jokes. The characters are drawn inexorably to the light of their own realisation: that teaching politics isn’t enough; that having a view of the world may not be enough; that we must take responsibility for our actions, or lack of them.

Oliver’s dark secret is one of causing death. His larger guilt has to do with the life he led, the betrayal of his wife, Philip’s mother. Nadia has escaped into a relationship with Philip because it’s the easy option. The war in Iraq is a flashpoint for decision-making at home. The Shavian scheme of the central body of the play is book-ended by two classroom scenes (Joseph Kloska and Wunmi Mosaku are the students) in which Nadia confronts complacency over American imperialism and despair over its foreign policy; how the latter replaced the former, in American public life, gives the play an extraordinary transatlantic cultural resonance.

It is possible that Oliver, the great seducer, wants to prise Nadia away from his own son. Instead, he succeeds in helping her articulate what she feels about our indifference to war, as well as to people. The play – as only drama can do – makes sense of global anxieties in the tiny detail of our own lives. And makes those lives seem important, too. This in itself is a wonderful achievement in a major, unmissable theatrical event.

- Michael Coveney