As the Beatles gathered for what would be their final live set on Jan. 30, 1969, they hadn't performed in public since Aug. 29, 1966 -- a three-year period in which the group would reach new artistic heights, even as it began to fall apart.

The reasons for their impending breakup were plentiful: the 1967 death of manager Brian Epstein; the striking musical development of George Harrison, who had been only a junior partner during the Beatles' early years; the inevitable maturation of the quartet, whose interests outside of the band ranged from the personal to the professional; and, of course, the drug use and egos, both spiraling out of control.

But their final live performance was unexpected and joyful as everyone gathered on top of Apple Records' rooftop in stodgy downtown London on a frigid January day -- especially considering what had been going on inside the Beatles camp over the past month. A documentary crew commissioned to film the group at work on a back-to-basics album project instead found the band disintegrating in front of the cameras.

The uncomfortable situation of being filmed during what was supposed to be a return to music-making didn't help matters. Over the previous 12 months, the Beatles had released a self-titled double album that couldn't have been more of a misnomer. 'The Beatles' rarely featured all four members in the same room at the same time, and when it did, they were simply backing up the principal songwriter, not truly collaborating. In a way, the high-concept and high-pressure follow-up project hastened the group's split.

The album and film, which were to be called 'Get Back,' would eventually emerge with a far more somber, and far more telling, title: 'Let It Be.' By the time the album finally reached the public in May 1970, the tapes had been sitting in Apple's vault for months after the original mixes (by engineer Glyn Johns) were rejected. Meanwhile, John Lennon had moved on, releasing the solo single 'Instant Karma,' which Phil Spector mixed. Lennon ended up handing Spector the stack of abandoned 1969 Beatles tapes, which the producer then reshaped through the prism of his Wall of Sound.

"He'd always wanted to work with the Beatles, and he was given the s---tiest load of badly recorded s--- -- and with a lousy feeling to it -- ever, and he made something out of it," Lennon told Rolling Stone's Jann Wenner in 1970. "It wasn't fantastic, but I heard it, I didn't puke. I was so relieved, after six months of this black cloud hanging over."

As with every other Beatles project up to this point, George Martin had overseen the initial 'Get Back' recordings. Along with Paul McCartney, and many other people, he was highly critical over the finished product, remarking that 'Let It Be''s credits should have have read "Produced by George Martin, overproduced by Phil Spector."

But the power of the group's rooftop concert yielded a trio of performances that ended up on 'Let It Be,' even if Spector smothered other songs like 'The Long and Winding Road' in layers of orchestral gauze. Nothing -- not inner-band disenchantment, not their subsequent neglect, not even Spector -- could buff out the glow surrounding the impromptu concert.

Still, the setting didn't do the Beatles any favors. The day dawned with a damp, bitter breeze cutting through the air, forcing both Lennon and Ringo Starr to borrow coats from their wives. The filming schedule for the documentary dictated that it all end with some sort of musical event. At this point, infighting among members had gotten to the point where no decision could be made. Both Starr and Harrison had quit the band recently, only to be coerced back. The group considered a few exotic locales (Egypt's pyramids among them), but in the end the Beatles found themselves on top of the open space nearest to them.

Everything came together very quickly, with staff members dragging equipment from the studio to the roof for the show. "You had to just use all the PA stuff that was at EMI and rig it so it was as loud as possible going down into the street," said technical engineer Dave Harries in the book 'Beatles' Let It Be.' "It worked quite well, actually -- quite surprisingly, considering." Alan Parsons, a young tape operator who'd later become a famous producer and leader of his own band, had to improvise when faced with the issue of wind noise. He went to a nearby shop to purchase women's stockings to cover the microphones. Parsons later remarked, "The store person thought I was either going to rob the store -- or cross-dress."

And yet something magical happened as the Beatles plugged in on top their Savile Row offices during lunch hour -- something that underscores the hope that they may have continued into a new decade had their early touring experience turned out different. Playing in the open air, away from screaming fans, Lennon and McCartney improvised with twinkles in their eyes, sharing remarkably unguarded smiles. They even rocked right through a visit from the police, who threatened to stop the concert over noise complaints from neighboring businesses. In short, they seemed emboldened by hearing themselves playing together again. The promise of the 'Get Back' concept finally came to fruition.

"It was really good fun because it was outdoors, which was unusual for us," McCartney remembered in 'The Beatles Anthology.' "It was a very strange location because there was no audience . . . so we were playing virtually to nothing -- to the sky, which was quite nice."

The performance, which lasted only 42 minutes, featured five takes on 'Get Back'; two versions of 'Don't Get Me Down,' scrapped because a loose and happy Lennon kept forgetting the words; snippets of several songs, including 'I Want You (She's So Heavy),' which was later expanded for its release on 'Abbey Road'; and 'I've Got a Feeling,' 'One After 909' and 'I Dig a Pony,' the three songs used on Spector's finished version of the 'Let It Be' album. Martin recorded the show on two eight-track machines in the Apple building's basement, with Johns and Parsons helping out.

Lennon, after the first stab at 'Get Back,' quipped, "We've had a request from Martin Luther." He was just getting started. After another take, he said, "We've had a request from Daisy, Morris and Tommy," before breaking into a brief rendition of 'Danny Boy.' McCartney joined in the fun, referencing the situation as the cops closed in to shut them down during 'Get Back'. "You've been playing on the roofs again, and that's no good, and you know your mummy doesn't like that / She gets angry / She's gonna have you arrested! Get back!" Finally, there's Lennon's timeless farewell at the end of the concert, preserved for eternity on 'Let It Be': "I'd like to say thank you on behalf of the group and ourselves, and I hope we passed the audition."

Fast-forward a few years, and the onstage concert experience had turned into big business. Stadium shows were now the norm, with technological leaps made in terms of amplification. Would hitting the road once in a while, and getting the Beatles away from the studio and their petty arguments, have saved the band? It's hard to say. But the rooftop concert certainly hints that the Beatles could have continued if they'd been able to play live together -- instead of laying down their individual tracks in an airless studio -- more often.

"When they were doing it, they were happy," said 'Let It Be' filmmaker Michael Lindsay-Hogg. "They actually kind of enjoyed it and had a lot of fun with it and each other."

McCartney, who bitterly complained about the Spector-tweaked 'Let It Be,' later oversaw a radically redone remix of the music in 2003 called 'Let It Be . . . Naked.' His treatment of the Beatles' final live performance would be a point of contention with some fans. He added 'Don't Let Me Down' to the lineup by editing together two different takes, but some of the show's spontaneity was lost. For example, the false start at the beginning of 'Dig a Pony' was deleted, as were Lennon's impromptu 'Danny Boy' at the end of 'One After 909' and, perhaps most jarringly, his "passed the audition" send-off.

The 'Let It Be' documentary, which hit theaters in 1970, was briefly released on home video in the early '80s. It's been unavailable ever since, even though there has been talk for years that a new release may someday hit shelves.