The sweet sadness of “All of Us Strangers” envelops the viewer with the warm melancholy of memory, which makes it the best “holiday” movie to have little to do with “the holidays.”

It’s a tenderly-acted romantic fantasy about a lonely man finding love in the fictive present and closure with his long lost parents, burnishing his recollections of them by meeting them in his mind and being tearfully surprised by how “they” turned out. Because they turn out to be pleased at how he turned out.

The Irish actor Andrew Scott, of TV’s “His Dark Materials” and “Fleabag,” plays Adam, a solitary 40something screenwriter mining his memory for a script in a new, mostly-empty high-rise apartment complex in London.

A fire alarm sends him outside, and that indirectly introduces him to Harry (Paul Mescal), a young neighbor who approaches him because gaydar is totally a thing and this nearly-empty building is making him a bit mental.

The tentative nature of their early relationship seems formed by their pasts. Harry has had intermittent contact with his family. Adam lost his parents at 12, in the ’80s.

“Car crash. Not the most ‘original’ of deaths,” screenwriter Adam admits.

“I’m sorry,” Harry says.

“It’s all right. It was a long time ago.”

“I don’t think that matters,” Harry replies.

And that’s the movie, that simple, sad, compassionate exchange.

Because as Adam takes the train south to the suburban city and suburban house where he lived as a child to jog his memory and add details to this script he’s writing, he stumbles into his father (Jamie Bell), still in his ’80s wardrobestill smoking and hitting the liquor store on the way home. He recognizes his kid and brings him home where the same late ’70s Ford Cortina sits in the driveway.

“Is it him?” Mum (Claire Foy) wants to know, as if they’ve just lost touch.

And thus begins a brittle and bittersweet reverie of visits, reminscing and catching-up. Adam will “come out” to them. One parent is more instantly accepting than the other. An awkward bit of treating him like a child passes, then Adam gets to bring them both up to date on the changes in the Western world’s tolerance and the end of AIDS. And his folks learn about how he grew up and what he became, and reconnect him with his past and his roots.

“You look just like my Dad,” mum gushes at one point.

“All of Us Strangers” is based on a novel by Taichi Yamada, and shows the same sensitivity writer-director Andrew Haigh has brought to his best work (“45 Years,” “Lean on Pete”).

He bathes his film in ’80s era “new romantics” Fine Young Cannibals/Pet Shop Boys era pop, underlining Adam’s “stuck at age 12” status just as subtly as “Guardians of the Galaxy” has its Star-Lord lean on his late Mom’s favorite mixtape.

Although the story traffics in a few cliches — drinking and drugs and dancing at the club, each partner wrestling with personal demons — the romance is treated with tenderness and respect.

And the parental reactions to this “discovery” that their boy grew up to realize he was gay features expected responses and an unpleasant unexpected one balanced against an utterly charming, idealized, best-case-scenario parental affirmation of love, connection and a wish for their child’s happiness and success.

Scott effortlessly conveys the guarded solitude, the alone-with-his-thoughts demeanor of the garreted writer’s lifestyle. He and Haigh take us on this character’s journey and make us relish the emotional release Adam feels as he “researches” a script and fantasizes and writes his way to a happier life.

Mescal brings a nice mystery to the forward, overly-chatty and troubled Harry, a most humane man who respects Adam’s boundaries even as he’s trying to express an interest in him.

Foy is spot -n as a mother rattled by this long-delayed “catch -up” with her son, a vulnerable and limited person whose character arc may be the film’s most rewarding and emotional.

And the once-and-always “Billy Elliot” Bell is a marvel, a working class Joe who probably wasn’t the best parent for a gay boy to have, but who — in Adam’s mind — was self-aware enough to know his limitations and offhandedly reconcile this side of him so that reconnect with his kid. Bell makes this character idealized but wholly credible, an EveryDad every kid would love “closure” with.

There are bigger films and more entertaining stories coming to screens this holiday season. But there isn’t one more life and love-affirming than “All of Us Strangers,” a movie that reminds us that memory burnishes loved ones for a reason. If we love them and remember them, they’ve earned it.

Rating: R, for drug abuse, sex, profanity

Cast: Andrew Scott, Paul Mescal, Claire Foy and Jamie Bell.

Credits: Scripted and directed by Andrew Haigh, based on a novel by Taichi Yamada. A Searchlight release.

Running time: 1:45