“Spencer” is nothing less than the reinvention of Kristen Stewart. Her portrayal of a troubled, mercurial, vain and bitter Diana, Princess of Wales is that startling, that much of a career-reset.

Her performance starts with an uncanny impersonation — the way Diana held her head, the whisper she always seemed to speak in, her carriage, stride and simple “ready for my closeup” beauty. We forget that this is actually the actress-turned-celebrity nicknamed “K-Stew” in a heartbeat. We even see flashes where she looks not just like Diana, but Naomi Watts, who played the princess in “Diana” some years back.

And almost as quickly as we lose the actress in that impersonation, she and the film transcend mimicry and plunge into the psyche of a woman wronged — a rich, powerful and unconcerned family that circled the wagons around the “outsider” to protect the feckless fop and heir to the throne, Prince Charles (Jack Farthing).

If I’ve seen a better performance in recent years than Stewart’s in this “fable from a true tragedy,” I can’t remember it. She’s stunning.

Pablo Larraín — he made “Jackie” with Natalie Portman a few years back — works from a detailed, minimalist screenplay by Steven Knight (“Locke,” “Dirty Pretty Things,” “Eastern Promises”) to produce an up-close-and-personal profile of Diana in her time of trial, the Christmas she reached her limit and ditch Charles and the Windsors, if not her fame.

Like “Jackie” and “Judy” and for that matter this month’s “King Richard,” this is biography as fantasia, a “what should have happened” story with a hint of fact and a whiff of fantasy.

It’s flattering, but nothing like a hagiography. There’s no image polishing, with just the barest mention of Diana’s “ban landmines” activism. She is vain, constantly asking “How do I look?” never venturing out less than stunningly turned out.

Diana is impulsive, lashing out within the strictures of her “duties” as she makes statements with what she wears and gives away what she “knows” about Charles and the old flame he never shook off, and carried on an affair with while both were married, simply by donning a pearl necklace.

Her most passive aggressive act of all is making the smug Corgi-fancier with “HRH” attached to her name, and her untidy family, wait. Stella Gonet has just a few scenes to suggest an Elizabeth that Helen Mirren won an Oscar portraying — emotionally-stunted, rigid and adamant about “tradition” and protocol, the older and more ludicrously out of date, the better.

When we meet Diana, she’s motoring about the countryside in her Porsche convertible, looking for Sandringham House, a drafty royal estate that is, coincidentally, next door to the great house gone to ruin that Diana grew up in, back when she was Diana Spencer.

Diana is so lost she has to ask directions from the gobsmacked inhabitants of a local pub. And failing there, she stumbles into the proud, dutiful and sympathetic royal head chef, Darren (Sean Harris, superb), who points her the right way.

Darren’s big staff prepares one ostentatiously sumptuous feast after another over the 1992 holidays, with him egging them on with a challenge borne of genuine affection and concern.

“I want our Princess of Wales to WANT something.”

Everybody in the Royal household knows of Diana’s eating disorder, and some even see it as a product of the ugly stresses of paparazzi, tabloid journalists, a husband straying with another woman, and his family’s indifference to Diana’s plight.

A sign in that kitchen orders one and all to “Keep the Noise to a Minimum. They Can Hear You.” And they do. For all the “security” surrounding this lot, the dressers, cooks and functionaries are — it is implied — their own gossiping/”reporting” social network.

Another sign is just as telling. The carefully-organized designer wardrobe Diana is to wear to every meal, outing, ceremony and the like carries a tag — “P.O.W.” You can think that stands for “Princess of Wales” if you like. But as we’ve already met the new Master of the Household, a retired Black Watch officer based on an RAF officer who had such duties, we can leap to a more ironic acronym conclusion.

Major Alastair Gregory (Timothy Spall, chilling) can seem sympathetic, but his sternness points to trouble on the horizon. He is there to keep the tabloids at bay, and it is implied, a tight rein on Diana.

“I watch so that others may not see,” he says, trying to curb Diana’s tendency to let the public and the press see more of her — candidly or otherwise — than the royal family would like, another way she fights her treatment and the restraints put upon her by her role, her fame and keeping her “place.”

Gregory cannot abide tardiness, and the admittedly paranoid Diana perceives cruelty and conspiracy in his actions.

Diana’s one confidante in the entire “holiday” travel party is her dresser, played by Oscar-winner Sally Hawkins. Her unguarded and improper bit of advice?

“They can’t change. YOU be the change.”

Over the course of three days, Diana’s “ridiculousness” — “silliness” is how she tells her oldest son William to describe it — rubs the mostly-offstage Royals the wrong way, time and again. We mostly see her, alone in her room or striding down cavernous empty halls, and see and hear servants of varying ranks knocking, calling out “Dessert, madam” or “The Family is waiting to open presents, madam.”

And Diana, finding a conspicuously-played book about the wife King Henry VIII murdered, Anne Boleyn, starts seeing Anne (Amy Manson) in her dreams and visions. Her paranoia and despair grow and grow.

Stewart’s portrayal is so vulnerable and alluring that if you’re so inclined, she could make you fall in love with “The People’s Princess” all over again. And it’s worth noting that the pretty actress has never been filmed in more flattering light. Kids who grew up on “Twilight” are thus encouraged to fall in love with Kristen Stewart again, too.

The movie around her is the damnedest thing, a script that ventures from cracker-jack to kind of crackers in the directions it takes Diana’s psyche and the lifeline it invents for her to grab.

“Spencer” owes a debt to “Jackie,” and to “Great Expectations” (a ruined family mansion, lost connections) and even “Citizen Kane.” Diana has her own “Rosebud,” and you’ll recognize it the moment she dons it.

Sure, it’s a one-sided portrait, although a more complex picture of Charles emerges despite the fact that Farthing (of TV’s latest “Poldark”) has few scenes to make an impression.

And no, it’s not the truth, or even The Gospel According to Diana.

But “Spencer” is still one of the best-written, best-acted pictures of the year. And if there’s any justice, Stewart will get the chance to smile her trademark coy grin and play with her hair, this time for a global TV audience. Oscar night could very well be her night.

Rating: R, for some language (partial nudity)

Cast: Kristen Stewart, Timothy Spall, Jack Farthing, Stella Gonet, Sean Harris and Sally Hawkins

Credits: Directed by Pablo Larraín, scripted by Steven Knight. A Topic Studios film, a Neon release.

Running time: 1:57