The so-called Lost Weekend, when John Lennon, from late 1973 through ’74, separated from Yoko Ono and relocated to Los Angeles, where he became a hard-drinking rock-club night owl while carrying on an affair with the 22-year-old May Pang (who had been John and Yoko’s assistant), has long been part of rock mythology. It’s been covered by everything from E! documentaries to Albert Goldman’s “The Lives of John Lennon.” Like many Lennon observers, I’ve always felt like I knew the basic bones of it.

I knew that John and Yoko, after marrying in 1969 and seeming like inseparable soulmates in art and life, began to have problems as a couple. That Yoko, trying to save the marriage, made the decision to set up John with May Pang, basically instructing the two of them to have a romantic affair. That in L.A., John, for the first time since the breakup of the Beatles (and maybe since the Beatles began), let his hair down and began to enjoy a more relaxed, fraternizing, at times carousing rat-pack existence. That he became a fixture at the Rainbow Bar & Grill on Sunset Boulevard along with Harry Nilsson, Alice Cooper, Bernie Taupin, Mickey Dolenz, and others who became known as the Hollywood Vampires. That after 18 months of partying and soul searching, John returned to Yoko, commenting at the time (in one of the wittiest quips of his life, which is saying something) that “The separation didn’t work out.” And that his decision to go back made the entire episode look like Yoko’s version of a Jedi mind trick.

The central figure in the new documentary “The Lost Weekend: A Love Story” is May Pang, who has told her tale many times (in her memoir, and on talk shows like “Geraldo” — in the film, we see a lot of clips of those appearances). The movie, directed by Eve Brandstein, Richard Kaufman, and Stuart Samuels, is told entirely from her point of view. It’s a portrait of the May Pang who grew up in Spanish Harlem as a second-generation Chinese-American (“a minority among minorities,” she says), and how she fell in love with rock ‘n’ roll and fell into fame with a kind of karmic destiny.

In the photos that we saw at the time of her and John, she always had a waifish beauty, and a certain mystery behind those tinted hexagon glasses. But in “The Lost Weekend,” we see that May Pang was a tough ambitious city girl, who spoke with a slight but blunt New York accent, and that after dropping out of college she had the chutzpah to talk her way into a job at the Apple Records offices on Broadway. She was a schmoozer, and when she began to work for John and Yoko, doing every makeshift task available — avant-garde film production assistant, costume designer — she had an ebullient smile and an easy-to-be-with vivacity. She was fun but circumspect (she didn’t drink or do drugs).

The documentary is Pang’s diary-like account of how the Lost Weekend played out, week by week, emotion by emotion, and on that score it offers a fascinating, revealing, and sometimes moving portrait of John Lennon unmoored, trying to find himself in a world that had caught up to him. The movie is also a portrait of Pang’s romantic passion, which as she portrays it was both innocent and deeply serious. To say that she was in over her head would be an understatement. She was 10 years younger than Lennon (and 17 years younger than Yoko), who was her boss and a Beatle. Once they were together, Yoko would phone her incessantly, wanting to know what was happening. It was all a lark; Pang was just going with the flow. But she relates, with a dailiness that’s convincing, how she and John became convivial and erotic companions, their affair rooted in a genuine affection and in Lennon’s discovery that he didn’t have to live in a way that was always so chained to his legend. (In the early ’70s, he’d become a real political scold; after the drubbing received by 1972’s “Some Time in New York City,” that was part of what he was letting go of.)

There’s amazing archival material throughout, and it gives you an unusually rich sense of what Lennon was like away from the limelight. The dark side is very much there. We hear Pang’s stories about how Lennon, in a drunken fit of confronting his demons, smashed up their place in L.A., and how he would hit her sometimes. And there are startling photographs that document the recording of “Rock ‘n’ Roll,” the album of early rock chestnuts that Lennon made with Phil Spector, who was entering his full mad-dog phase. But according to Pang, the fabled tales of Lennon’s misbehavior, as when he and Harry Nilsson, after too many Brandy Alexanders, got kicked out of the Troubadour nightclub for heckling the Smothers Brothers, were more the exception than the rule.

The narrative that shapes “The Lost Weekend” is May Pang’s gently building insistence that she and Lennon were truly in love. And we have to take that on faith, since it’s all very subjective and not necessarily borne out by what happened. Did Yoko really set the whole thing up? According to May Pang, she totally did, walking into Pang’s office at the fortress-like Dakota, where John and Yoko had moved to feel more secure (Lennon, at that point, was being seriously harassed by the FBI, since President Nixon wanted him deported), and basically giving her an executive order: You’re going to have a relationship with John. Yoko had observed John’s infidelity, so she figured that she would let him stray with a woman she could control. It was, by any standard, a decision of seriously kinky manipulation.

Yet this was the bed-hopping, do-what-you-feel ’70s, so it all seemed a little less weird at the time. It wasn’t Yoko’s idea that the two of them move to L.A.; that was John’s impulsive decision. The documentary chronicles how after about a year there, they returned, just as impulsively, to New York, moving into a small apartment on E. 52nd St., where they lived through the first months of 1975. We see Pang’s photograph of Bob Gruen snapping his famous photograph of Lennon in a New York City T-shirt. One night, she and John saw a UFO from the rooftop (Lennon’s description and sketch of it are haunting), and according to Pang they were talking about buying a house in Montauk.

But Yoko had already re-entered the picture, showing up backstage to see John at the premiere of an Off Broadway show based on “Sgt. Pepper.” There are moments in the film when Yoko, to say the least, does not come off well — notably in Pang’s description of how Yoko attempted to cut off Lennon’s relationship with his son, Julian. Julian is interviewed throughout the film, and he (like his mother, Cynthia) maintained a close bond with Pang. That Pang helped to bring John and Julian back together, despite Yoko’s machinations, seems more convincing than not.

What doesn’t seem convincing, at least as the film presents it, is the final twist in this extraordinary rock ‘n’ roll soap opera. After John, seemingly out of nowhere, goes back to Yoko, and Pang confronts him about it, he says, quite simply: She’s letting me come back. Letting him? That doesn’t square with what the film has implied — that Lennon had drifted away from Yoko. His comment suggests that their separation was always contingent on an understanding between them. But that’s something we’d have to guess at, since the life of John Lennon remains, for all the ways it’s been chronicled, not quite knowable. “The Lost Weekend” is a compelling movie and a valuable puzzle piece, but it’s only pretending to be the whole puzzle.