Within a few weeks, American moviegoers will be given the chance to wallow in the glitz, glamour, and guillotines of Sofia Coppola’s “Marie Antoinette.” For now they will have to make do with a dowdier, more discreet queen, the one who has been reigning in England for more than half a century now, a monarch who shows every sign of hanging on to her crown and, thankfully, the head on which it sits.

In all the decades of Elizabeth II’s painstakingly (and sometimes painfully) dutiful, conscientious, and, yes, tenacious reign, there has only really been one brief, bizarre period, of just about a week, when there was the slightest danger that the Windsors might, like so many of their less fortunate relatives in so many less fortunate countries, be asked to pack their bags. It’s that interlude, the disturbing, absurd and even slightly frightening days that followed the death of Princess Diana that is the focus of “The Queen,” a compelling new docudrama by British director Stephen Frears that opens the 44th New York Film Festival on Saturday. It’s easily the best film I’ve seen this year.

From that tunnel in Paris to Earl Spencer in Westminster Abbey, these events are still so familiar that Mr. Frears is left free to concentrate on the most interesting aspect of the story: the plight of a monarch at bay, puzzled, hurt, and confused by the behavior of a nation, her nation, that appeared to have changed, almost overnight, beyond all recognition. The quiet, disciplined, loyal, stoical Brits of the Queen’s youth, of the Blitz, of so much more, had vanished, to be replaced by a volatile, hysterical, and vindictive mob caught up in a self-indulgent bacchanalia of grief for a princess they never really knew. Suddenly Elizabeth’s virtues — restraint, self-control, that famous sense of duty — had come to be seen as vices by a population baying for her to show that she “cared” by faking tears over the death of the more “genuine” Diana.

The movie itself begins about four months earlier, setting the scene with Tony Blair’s 1997 landslide election victory and then the first audience between the novice prime minister (a puppyish Michael Sheen) and the veteran queen (Helen Mirren), coolly charming, intimidating, and on top of her game. It’s beautifully observed and very funny (Peter Morgan’s script is a consistent delight, meticulously researched and, I suspect, largely accurate), but Dame Helen really comes into her own (Oscar! Oscar!) as events begin to engulf the embattled monarch.

Helped by the hairdo that launched a million stamps, her own surprisingly strong facial resemblance to the Queen and, dare I say it, more than a little padding (there goes my knighthood), the former Inspector Tennison turns out to make an uncannily realistic Elizabeth II. More precisely, Dame Helen plays a woman playing the Queen, an approach that goes a long way to explaining why she is so remarkably convincing. Monarchy is a performance. The Queen’s tragedy is that it’s a role she almost certainly never wanted. The Queen’s genius is that she does it so well.

Nevertheless, watch Her Majesty carefully enough (as many of us English tend to do) and it’s just possible to detect that the smile, the wave, the small talk, and all the rest of it are acts of will, the work of an actress, a pro, trapped in a role that will last a lifetime. And in her performance, Dame Helen catches this perfectly. Every now and then she lets glimpses of the real Elizabeth, that long-vanished Lilibet, peep through, and then, abruptly, deliberately, the face freezes, the mask is put back on, and safe, comfortable distance returns.

When, in the middle of the crisis, she lets her guard drop just enough to ask her mother (Sylvia Syms, who rather surprisingly chooses to portray the Queen Mum as a sitcom gran) for advice, that advice is, like that of her husband (James Cromwell playing Prince Philip as a caricature of himself, which of course, he really is) absolutely hopeless. An agonized, nervous Prince Charles (Alex Jennings, splendidly twitchy) and her principal adviser have the right instincts, but are too intimidated by her to do much good. The old pro is, she discovers, isolated, alone, adrift. The script no longer works, and the audience is out of control.

Eventually, help, and a new scriptwriter, shows up in the shape of an increasingly assured Tony Blair, a master politician with an instinctive understanding of the new Britain and, critically, what the royals would have to do to win back public favor. He’s the self-proclaimed modernizer on a mission to transform what was left of the Queen’s old England, but he’s also astute enough to want the monarchy to survive, and, despite the gibes of his colleagues and his wife, fair enough to appreciate all that the Queen had done for her realm.

As for the woman whose death triggered the whole crisis, her image flits and flickers through the movie in clip after clip of archive footage, the only one of the film’s protagonists not to be played by an actor. It’s a clever device: It adds to the sense of authenticity and serves also as a pointed reminder of just how much that lost princess was fantasy, creation, accomplice, and victim. The texture of that footage — faded, grainy, herky-jerky, recognizably different from the rest of the film, almost ghostlike in impact — only serves to underline that Diana had gone, never to return.

In the end, as we all know, the fever broke. The film concludes as it began with Mr. Blair visiting Buckingham Palace. No longer the novice, he is confident, too confident, perhaps, a politician at the peak of his popularity, and angling, maybe, for a word of thanks for all his help. What he gets instead is a warning on the fickleness of our age. “One day,” the Queen says, “quite suddenly and without warning, the same thing will happen to you.” And now, of course, it has. Meanwhile, the monarchy itself endures and Diana’s memory fades. On the fifth anniversary of her death, one writer noted that the gardens of Althorp (the Spencer family home) and Kensington Palace were “deserted.”

“The public,” he wrote, “had moved on. They were now too busy ‘never forgetting’ other people.”