Berlin Hanover Express, Hampstead Theatre, London

The express train ran from Berlin to Hanover past the death camps. Do you stay on? Or jump off and bear witness? That's the question at the root of TV writer Ian Kennedy Martin's debut stage play, a powerful slice of period drama set in the Irish consulate of Berlin in 1942.

Two Irish officials, Mallin and O'Kane, are supposed to be maintaining political neutrality while sleuthing the files of a former colleague who might have passed on sensitive information – like the truth of what's happening – to the British. The nearly intertwined flags of the Irish Republic and the Third Reich over the door convey the official political line. But turning a blind eye becomes increasingly impossible as a German security officer, formerly maître d' at the Kempinski Hotel, stalks the legation's Polish cook with questions about her dissident brother and family origins. Mallin sticks to his guns, but O'Kane is wavering.

The chill statement that "a beautiful body is wasted on a Jewess" sums up the horror of the play's central scene, as the cook, played with ferocious dignity by Isla Carter, responds to the disgusting commands of the officer by striking defiant pornographic poses. Like Isabella in Measure for Measure, she believes she is saving her soul, and her brother's, by yielding her body. And like Isabella, she is deceiving herself.

It is a hard scene to watch, and the actors, and director Michael Rudman, spare us nothing. It is a fact that the new Irish Republic allowed the Germans to park U-boats on their shores, and that Pope Pius XII knew about the death camps. Hindsight makes it worse, but Sean Campion's finical, bespectacled Mallin, shuffling papers in docile compliance, at least runs up a flag for the idea of solidarity between two new European national enterprises.

His sidekick O'Kane, brilliantly and ebulliently played by Owen McDonnell, boasting of family connections to Eamon de Valera while doing a soft- shoe shuffle to "Deutschland über alles", is wising up like his predecessor, leaving Mallin to share common ground with Peter Moreton's bull-like officer.

An intriguing and entertaining play, meticulously directed, it might benefit from 10 minutes lopped off, specifically the extraneous speech about Romans. Otherwise, a palpable hit.

To 4 April (020-7722 9301; www.hampsteadtheatre.com)