Of course it didn’t happen this way. Not exactly. Not even remotely.

“A Royal Night Out,” about VE-Day, 1945, when the two British princesses, Elizabeth and Margaret, slipped out of Buckingham Palace, “incognito,” and experienced the greatest celebration London had ever seen, is wish fulfillment fantasy.

But it’s bloody adorable.

We’d love to think those stuffy, in-bred royals cut loose, experienced a little of the common life. They’d flirt, dance the Lindy Hop, knock back a beer or two, and some champagne. They’d be treated like any other single young women present at a night-long baccianale.

The horror!

At its best, this Julian Harold (“Kinky Boots,””Becoming Jane”) comedy is a throw-back, a madcap romp set to a swinging, big band beat. The Brits apparently didn’t go for it, the sticks in the mud. They’re missing a laugh-out-loud riff on those Windsors they worship, before QEII discovered her fetish for hats.

Elizabeth is “Lillabets,” here, a dutiful daughter played with spirit and spunk by Sarah Gadon (“Spider-Man 2,””Dracula Untold”). Margaret is “Mags,” or “P2” as she quips, on occasion (Princess 2). Bel Powley (“Diary of a Teenage Girl”) brings a taste of hellion to her.

Which is just so…right. Bad decisions? She was infamous for them. Surely she started in her teens. She nags her sister into asking her parents to let them go out and experience the joy of a nation relieved from six long years of fighting the Nazis.

With every idiotically elongated vowel she utters as Mags, Powley is a stitch.

“Best be-HAAAAAVE-your. HONestly.” Her every greeting, drunk or sober, is worthy of Madea.

“Helll-AAAWwwwwwwwww!”

The girls get permission, dodge the underhanded formal party among aged nobility that their mother (Emily Watson, no fun in all the right ways) springs on them. And that’s how they lose their officer corps escorts, who figure they’re free to hit the brothels and the pubs until dawn. Until they drunkenly realize they’ve lost the lasses and will have to face their imposing father after he’s given another “King’s Speech” (Rupert Everett, nicely done).

“Lizzie” has no money and is rescued by a cynical, AWOL bomber gunner (Jack Reynor), a bloke who’s over the whole class system and the Royal Family. He doesn’t know who Lizzie is, but “Little Miss Tofeenose” has no business saying “My whole FAMILY served in this war!”

“Sandwiches. Maybe.”

It’s a comedy of goofy little grace notes — the two escort officers (Jack Laskey, Jack Gordon), drinking to their doom, singing a glorious duet of “God Save the King” in a toilet, Mags being slipped a spiked drink by a Naval officer who sweeps her into a brothel, only to see her rescued by the royalty-loving hustler-smuggler (Roger Allam, hilarious) who runs the joint.

“P2! In my knocking shop!”

Mags, drunkenly slurring “We’re GERMAN, you know. Mustn’t talk about it” about her Hanover-turned-Windsor family, is again, just so right.

The surprises are few, and none of it am0unts to a whole lot. But for those up to taking yet another British sentimental journey to “their finest hour,” “A Royal Night Out” manages something unheard of in the decades of Windsor wooliness since. It makes them cute, if only for one night.

MPAA Rating:PG-13 for some sexual content and brief drug elements

Cast: Sarah Gadon, Bel Powley, Rupert Everett, Emily Watson, Jack Reynor
Credits: Directed by Julian Harold, script by Trevor Silva and Kevin Hood. An Atlas Distribution release.

Running time: 2:36