Brian Wilson‘s never had much of a poker face. And the decades, the long battle with mental illness, hasn’t changed that.

In “Brian Wilson: Long Promised Road,” the Beach Boys icon and Hall of Famer can be frank and unguarded, even when he’s drifting into “scared” and anxious. Pushing 80, he forgets things he’s just heard but remembers an awful lot from “way back when,” many timelines of “way back when.”

Gently questioned, catered to, flattered and indulged in this uplifting and sweet-spirited documentary, he’s an inspiring sight to anyone who knows what he’s been through and knows others going through mental struggles.

“Long Promised Road,” taking its title from a lesser-known tune by the band, is a celebration of the glorious third act of a performer whose struggles became legend, whose victimhood became notorious and whose “genius” no longer requires quotation marks.

As the movie makes plain, his “auditory hallucinations” and anxiety attacks manifested themselves early, forcing him to quit touring just as his band was reaching its peak. So he threw himself into “turning the studio into another instrument” in the ensemble, and in a friendly, informal rivalry with The Beatles, produced some of the landmark albums of the 1960s, famous for their aural invention and musical complexity.

He fell into a spiral as the band’s relevance faded, a deadly blend of mental illness and drugs that could easily have killed him and made his problems sad tabloid fodder.

Then, in 2004, coming out the other side of his schizoaffective disorder and the loss of confidence that his relations with the band and an album he hadn’t finished brought on, he took up touring again — with a vengeance. And the concerts became love-ins, the music he started recording again was critically acclaimed.

Filmmaker Brent Wilson (no relation) follows Wilson and a friend, Rolling Stone journalist Jason Fine, as they take us into the routines of his life these days — meals at a familiar deli, studio time with his longtime band — and on a road trip around the Southern California landmarks of his life.

And fans, from Elton John to Nick Jonas, Springsteen to legendary producer Don Was, sing his praises.

“He had an orchestra in his head,” Elton John marvels. “The Beatles had George Martin to do (orchestrations, arrangements and producing) for them. Brian just had himself.”

Brian and the band’s masterpiece, “Pet Sounds?”

“In terms of musicality,” Bruce Springsteen opines, “I don’t think anybody’s touched it, since.”

Don Was, who has worked with Wilson in the studio, sits at a mixing console and hears a little of “God Only Knows.” “I don’t know what that is,” he says, confused about exactly what instruments were used in this arrangement. “Flutes? With reverb?”

That’s the first of several jaw-dropping musical moments in the movie, that one of the most storied music producers of our time can’t figure out what this “kid” of 22-24 was doing in a 1960s recording studio back in the analog era.

A half century of 1960s cheesy to achingly candid later TV interviews are sampled here. But the clever hook this film hangs on is seeing Wilson today, with a most sympathetic interviewer — Fine — driving him to Paradise Cove and Laurel Way, to old houses and bits of Brian Lore, setting the record straight and peeling away the image, gossip and fiction from his story.

“Everyone says you stayed in your bedroom for years,” Fine says of Wilson’s most infamous crack-up. “You didn’t do that.”

“Nooo. Just a couple of weeks.”

We see Wilson’s house back in the days when he made his music room a “sandbox,” literally — with a piano parked in the middle of the sand. In another room, he kept an Arabian Nights tent.

“What’d you do in there?”

“Smoked grass, ate peanut butter and jelly sandwiches.”

We see every hint of nostalgia, pain and regret cross Wilson’s face, just sitting in the front seat of Fine’s SUV. Waves of anxiety hit him without warning.

“What do you do when you get scared?” he asks his friend.

Fine probes the relationships Wilson had with his tyrannical dad Murry and controlling psychotherapist Eugene Landy, and gets the eldest Wilson brother to open up about his late bandmate siblings Dennis and Carl. Brian tears up upon hearing Carl’s vocals on the Beach Boys collection Fine keeps on his car stereo and the film cuts back back and forth between Carl’s concert rendition of “God Only Knows” and Brian’s version in concerts in recent years.

And when Brian reveals he’s never heard drummer-surfer-brother Dennis’ “lost jewel” solo LP “Pacific Ocean Blue,” Fine treats him to a bit of it.

“Wanna hear more?”

“I wanna hear it ALL!”

“You were really good friends with Dennis…”

“Because we used to snort cocaine together. He’d buy me cocaine…I like his song, ‘Forever,’ so beautiful…Big heart.”

The concert footage here reminded me of the warm Glen Campbell documentary, “I’ll Be Me.” Wilson fills the stage with musicians, and while he’s in fine voice and right at home, you get the feeling they’re there to ensure that everything comes off perfectly — supporting, admiring, protecting and performing with him all at the same time.

The portrait that emerges in “Long Promised Road” is that of a gentle soul who’s never been anything less than an open book. When he hears from Fine that Jack Rieley, the manager who brought the band back to relevance in the early ’70s before an acrimonious split, died a couple of years before, Brian breaks down into tears.

He is effusive in complimenting prickly cousin and lead singer Mike Love’s voice, and muted in his criticism even of those who wronged him.

And you realize that maybe the film biography of a few years back, “Love & Mercy,” didn’t wholly do him justice because really, who would believe anybody could be this gentle, upbeat and a bonafide genius to boot.

Rating: unrated, drug use discussed, profanity

Cast: Brian Wilson, Don Was, Bruce Springsteen, Linda Perry, Gustavo Dudamel, Melinda Wilson, Nick Jonas, Elton John, Jason Fine

Credits: Directed by Brent Wilson, scripted by Jason Fine, Brent Wilson and Kevin Klauber. A Screen Media release.

Running time: 1:33