“— we already had Dennis Wilson,” interjects Mike Love, the lead vocalist and cousin of the Wilson brothers Brian, Dennis and Carl. “Nepotism won the day.”

Johnston continues: “I came back from dinner one evening to find John Lennon and Paul McCartney in the suite, waiting to hear Pet Sounds. They loved it, made me play the album twice, and said the five vocals on Wouldn’t It Be Nice helped them write Here, There and Everywhere.”

“Then there was Sgt Pepper,” Love adds. “We did the cover shoot for Pet Sounds at San Diego Petting Zoo, and when I was in India at the Maharishi’s place [Rishikesh, 1968] Paul McCartney said, ‘Mike, you really need to take more care of your album covers.’ I was somewhat chagrined so I said, ‘Paul, we’ve always been more concerned with what’s in the sleeve than what’s on the outside.’ Well, I had to come up with something.”

I meet Love and Johnston, both of whom are still touring as the Beach Boys, in Studio 2 at Abbey Road, the very place where Sgt Pepper was recorded. At 83 and 81 respectively, they come across as two amiable, baseball-capped, all-American geezers reminiscing about the good old days as The Beach Boys, a documentary by the veteran Hollywood producer/director Frank Marshall, is released — alongside a coffee table book telling the band’s story. And that story turns out to be far more complex than their image as the embodiment of an endless California summer suggests.

This month, after five decades of mental health issues and the death of his wife, Melinda, in January, Brian Wilson was placed under the conservatorship of his manager and publicist. “We saw him last September, in Paradise Cove,” Love says, referring to the Malibu beach where in 1962 the Beach Boys posed in a hot rod for their first album, Surfin’ Safari. “He doesn’t get around so well any more, but we sang Their Hearts Were Full of Spring and Fun, Fun, Fun. Brian was remembering things we did at high school, and it was heartwarming. People have always come between Brian and me, and divergent lifestyles didn’t help, but when we got together it was love and harmony.”

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That happened because Marshall insisted on reuniting the surviving Beach Boys for the final scene in his movie. “It was the first time the others had seen Brian in a long while,” he recalls. “It came out of me trying to tell all sides of the story and gaining their trust. They placed themselves in my hands.”

The Beach Boys started out as a family hobby, with Love, the Wilson brothers and a childhood friend, Al Jardine, sharing a love of the harmonies of the Four Freshmen and the teenage rock’n’roll of Chuck Berry. “We were close back then,” Love says. “We were spending so much time together — for my senior year at high school I invited Brian to go as my guest on a trip to Catalina [Santa Catalina, an island in California] rather than a girl. Brian’s mother, Audree, had a fantastic musical ear, which her sons inherited, while his father, Murry, was more of a salesperson, which helped us get established. But Murry was not very kind to his sons, or to his nephew — that’s me.”

He certainly wasn’t. Murry was a frustrated songwriter who poured his ambitions into his offspring and tales of his violent outbursts are legion. Brian claimed that Murry once hit him over the head with a lead pipe so viciously that the hearing in his right ear was damaged permanently. It was the middle brother, Dennis, who got it worse, though.

“Dennis was athletic, handsome: the girls loved him,” Love says of the only actual surfer in the Beach Boys until Johnston came along. Dennis nullified the torments of his childhood with drugs and booze, which led to him drowning in 1983 after an all-day drinking session. “He was a lot like his dad. While Murry was abusive to Brian, and Carl stayed out of the way, Dennis fought back.”

“I’ve dealt with a lot of stage mothers and fathers who live out their dreams through their kids and Murry was one of them,” Marshall says. “‘I’m a genius too’ was his line. It was jealousy.”

The Beach Boys features audio footage of Murry berating the band in the studio for not following his orders and telling them to “humble up”. In 1964 they sacked him, which led to a lifetime of legal woes, not least because in 1969 Murry sold the Beach Boys’ publishing company, Sea of Tunes, against their wishes for a paltry $700,000. He died four years later, at 55, having neglected to credit Love as the lyricist for 79 Beach Boys songs. That led to Love suing Brian, who co-founded Sea of Tunes, in 1992, beginning a string of lawsuits that ensured any harmony between the rival Beach Boys was, for a while at least, confined to the studio.

“The sale of the publishing devastated Brian and caused me loads of trouble,” Love says. “Brian was so gifted they called him ‘the genius’. But he wasn’t adept at lyrics. That was my role. And I wasn’t credited by Murry.”

“I was there when [the top LA session musicians] the Wrecking Crew were recording California Girls with Brian,” Johnston says. “Brian shouted, ‘I need some words.’ Mike wrote some down on a legal pad and two hours later he had lyrics to one of the greatest Beach Boys songs of them all.”

In 1966 Love came up with the lyrics to Good Vibrations while driving his Jaguar XK-E on the way to the studio, after Brian had been working on the psychedelic pop masterpiece for four months. “The spontaneity of the words, driven by anxiety that I hadn’t done them yet, combined with Brian’s well thought out music, made it work,” Love says. “Last year a psychologist from Sheffield did a study on the songs that make people feel good. Good Vibrations was No 1.”

After that high point, the vibrations were anything but good. Love has been cast as the guy who wanted to stick to the old surfing-and-girls formula while Brian headed increasingly toward experimental innovation, but the truth is more complex. In 1967, with the rest of the band on tour, Brian hooked up with Van Dyke Parks, an erudite but abstruse songwriter, for a kaleidoscopic vision of old America called Smile. What Brian intended as “a teenage symphony to God” was never completed, marking the point at which its composer fell apart.

“I thought the music was brilliant,” Love says of the lost masterpiece, which would have featured such beautiful songs as Cabin Essence, Heroes and Villains and Surf’s Up, complex, thoughtful fan favourites that eventually appeared on later Beach Boys albums. “But I couldn’t relate to the lyrics. I love poetry, but I also like things to make sense, so I came up with the term ‘acid alliteration’ to describe the words on Smile.”

There was also his cousin’s mental disintegration to deal with. “The incursion of drugs was the most negative thing that ever happened to our group,” Love says. He discovered transcendental meditation around the time Brian was discovering LSD. “Brian shelved Smile after thinking a bit of music he wrote called Fire caused an actual fire down the street. You know, we all participated in singing on those tracks, but some of the things happening around then were a little too weird for me. And the reality is, even in the touring group there was the smoking jet and the non-smoking jet. I don’t need to elaborate on what was being smoked.”

“I never smoked and never took drugs,” Johnston announces. “Instead, I went surfing. Still do.”

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All this created a schism: Brian in the studio with a hip new crowd; the rest of the Beach Boys out on the road, playing the hits. “They were square,” Marshall says. “Striped shirts, happy sounds … They were not recognised as a serious group. I think it is Al Jardine who says in the film, ‘We wanted to be Beach Men.’ But they installed a studio in Brian’s house so he could keep experimenting, which shows the respect they had for him. The problem was that Brian was so whacked out.”

I have interviewed Brian three times. The first, at his house in Hollywood Hills in 1999, wasn’t too bad. Encouraged by Melinda, he talked about the impact hearing Be My Baby by the Ronettes had on him, how LSD “knocked me for a loop” and how Smile was too far ahead of its time. On our second encounter, in 2018, the longest sentence I managed to extract was, “Can I get a glass of water?” By the third interview, in 2021, Wilson’s eyes were filled with some unfathomable fear and sadness and even getting a one-word answer proved impossible. In desperation I asked what he liked to order at his favourite deli. “Food,” came the reply.

I wonder how Marshall got anything out of him. “It was difficult,” he says. “I would ask, ‘What was the best thing about Dennis?’ ‘He was a singer.’” Was it all a ploy by Brian, a way of keeping people at bay? “I thought that, but then we took a break in the kitchen and he was the same. I couldn’t have a conversation with him, to be honest. He had fried himself.”

“Brian had LSD-ophrenia,” Love says, when I ask him at what point the schizophrenia Brian was finally diagnosed with in 1984 became apparent. “Brian is extremely sensitive and he’s excessive about everything, which is great with music, but smoking? He would get through four packs a day.”

In 1975, Brian’s first wife, Marilyn, and the rest of the Beach Boys hired the controversial psychologist Eugene Landy to treat Brian. Landy ended up becoming his business adviser and co-songwriter, while cutting off contact with the rest of the group and engaging in some extremely unconventional practices. Making Brian eat spaghetti off the floor was one of them.

“We had to intervene with the nefarious Dr Landy because he took over Brian’s whole life,” Love says. “He wouldn’t even let his mother talk to him. But he got Brian down from 300 pounds to 185, so he did something right.”

That was after the Beach Boys produced a run of fantastic albums at a time when their stock was never lower. In 1971 Surf’s Up addressed the limits of their old happy-go-lucky image with deep pathos and tackled the emerging issue of environmentalism on Love’s Don’t Go Near the Water. Was it a deliberate decision to capture the mood of the era?

“I think it was conscious,” Love says. “We did a song called Student Demonstration Time, which is as relevant today as it ever was. We had the Vietnam war going on then and we’re still having wars today. Humanity still cannot comprehend the idea of instant karma.”

For Marshall’s part, making the film taught him about the innovations of a band he had not taken particularly seriously. “I can’t think of another group where one member is in the studio composing while the rest are out touring,” he says. “I knew about the harmonies. I didn’t know how important or influential the Beach Boys were.”

That influence stretches all the way from the Beatles to Beyoncé, who included lines from Good Vibrations in Ya Ya, on her latest, country-themed album Cowboy Carter. I ask Love and Johnston if they can imagine making music with Brian again.

“We should do some music therapy with Brian,” Johnston suggests. “He can’t bust a move, but he might just bust a hit.”

“As long as he is alive,” Love says, “Brian will be able to sit at the piano and do what he does. He has medical supervision, he communicates with his daughters Carnie and Wendy, and you know what? We lost Dennis in 1983, Carl in 1998, and somehow the music has lasted. When Brian and I do get together, it really is love and harmony. And we wouldn’t have done the Beach Boys without each other.”

That sounds like something we never thought we’d see in the knotty world of the Beach Boys: a happy ending. The Beach Boys streams on Disney+ from May 24

The Beach Boys in pictures

The Beach Boys by The Beach Boys, the new limited-edition book published by Genesis which tells the story of the band through their own words and pictures, is now available to order from thebeachboysbook.com