Buchenwald wasn’t so bad.”

If there is a single line that stays with you for days after this mesmerising evening, in which 84-year-old Maggie Smith returns to the stage after eleven years to play 102-year-old Brunhilde Pomsel, Goebbels’s secretary, then this is probably the one.

In a different context it might be an example of Jewish irony — a Holocaust survivor’s gallows humour, perhaps. The line has something of that quip attributed to Walter Matthau about it. There are several versions, but one is that Matthau was staying in Krakow with his wife Carol and Matthau asked the concierge of his hotel to book a cab so that they could visit Auschwitz. He and his wife waited, and waited but the cab never came. So Matthau called the concierge who apologised profusely and confessed he had forgotten to book the cab which was when Matthau said, “Okay, but I gotta tell you, you’ve ruined Auschwitz for us.”

There is, however, no irony when Pomsel says her line about Buchenwald. It is more a statement of fact, not about the Jewish experience of the camp, but hers. This was after the war, she explains, where she was held after being politely interrogated by the Russians. In her Buchenwald there was barley soup three times a day and a theatre with a real orchestra pit. Sachsenhausen, the other former concentration camp to which she was later transferred, was also relatively comfortable.

But we are getting ahead of ourselves. Christopher Hampton’s one-woman play is based on a one-woman film, a documentary by a Viennese collective in which the real — now late— Pomsel describes her war, her life before it and after it. During its various stages her career as a secretary she worked “for a Jew in the morning and a Nazi in the afternoon”, for German radio and then for the Propaganda Ministry and its boss Joseph Goebbels. Goebbels could be attractive in his way, but was a bit of a cold fish, apparently.

As Pomsel, Smith relates all this with a remarkable display of naturalistic acting. In Jonathan Kent’s simple production she commands the stage for an uninterrupted one and a half hours simply by sitting in a living-room chair and talking.

I suppose it could be argued that Smith’s innate charisma and ability to inflect a line of dialogue with wry judgment, might undermine Pomsel’s sheer ordinariness. Because that, after all, is the point of a play that subtly reveals what complicity actually looks like.

And it looks nothing like the way we think atrocity normally looks. Here it is utterly unremarkable and a mostly friendly thing which expresses sympathy for victims, but which looks the other way when its victims are in sight. Complicity, it turns out, is merely a question of not asking questions.

This was relatively easy for Pomsel though as there was “no persecution of the Jews. Everything was fine.” Well, Jewish businesses were boycotted. And true, Jews around her disappeared, including her good friend Eva Löwenthal — very pretty, beautiful eyes, not very tall, reddish hair, delicate features but she did have that Jewish….Pomsel finishes the sentence as discreetly as she can with hooked finger placed next to her nose.

And also true there were her neighbours, the Levis, whose daughter Hilde, Pomsel grew up with. And there was that awful thing, what was it called — Kristallnacht.

But the point is Pomsel cannot be made to feel guilty. She sat at her desk in the Propaganda Ministry but really and truly she knew no more than the local greengrocer.