Directed by Stephen Frears from a very smart script by Peter Morgan, who helped write “The Last King of Scotland,” also about crazy rulers and the people who love (and hate) them, “The Queen” pries open a window in the House of Windsor around the time of the death of Diana, Princess of Wales, blending fact with fiction. It begins just days before her fatal car crash in 1997, when the princess, glimpsed only in television news clips and photographs, had completely transformed into Diana, the onetime palace prisoner turned jet-setting divorcée. The transformation was fit for a fairy tale: the lamb had been led to slaughter (cue Madonna’s “Like a Virgin”) and escaped in triumph (crank the Material Girl’s “Bye Bye Baby”). Elizabeth II wore the crown, but it was Diana who now ruled.

How heavy that crown and how very lightly Helen Mirren wears it as queen. With Mr. Frears’s gentle guidance, she delivers a performance remarkable in its art and lack of sentimentalism.