Up until its official arrival on Oct. 31, 2011, the Beach Boys' Smile was one of music's most famous unreleased albums.

Bootlegged, pored over and reconstructed for more than 40 years, the record was said to be Brian Wilson's masterpiece, a reaction to the Beatles' culture-shifting period that yielded both Revolver and Sgt. Pepper's Lonely Hearts Club Band. It was a mutual game of one-upmanship that also included the Beach Boys' previous album, and Wilson's true masterpiece, Pet Sounds.

But somewhere along the line, things derailed for Wilson, his group and the album.

The year or so leading up to the release of Pet Sounds in May 1966 was a tumultuous one for Wilson and the Beach Boys. In late 1964, he freaked out on a flight from the band's hometown of Los Angeles to Houston, where they were scheduled to perform. Always a bit mentally fragile and not comfortable in the public eye, Wilson retired from touring and focused his efforts on shaping Beach Boys albums into meticulously produced and orchestrated works of art, modeled after the records made by his idol, Phil Spector.

Pet Sounds became his obsession after hearing the Beatles' Rubber Soul, their first real grown-up album. He labored over how Pet Sounds sounded, using studio veterans to help him realize the musical visions in his head. It exhausted him, as well as the other members of the Beach Boys, who weren't quite sure where their resident genius was headed.

But they followed him anyway. Near the end of the album's sessions, Wilson began to construct what he considered to be the start of his next opus in the form of "Good Vibrations," which was released as a single in October 1966. It was a weird little song, divided into sections, with nontraditional pop instruments like harpsichord, cello and, most strikingly, theremin shoved into the busy mix. It paid off, giving the Beach Boys the biggest hit of their career.

That was just the start. Things got a whole lot weirder from there.

Listen to the Beach Boys' 'Good Vibrations' From 'The Smile Sessions'

"Good Vibrations" marked the start of Smile, the Beach Boys' next proposed album, one that Wilson referred to as a "teenage symphony to God." For 10 months, Wilson worked on Smile, building up and breaking down more than a dozen songs slated for the LP. Countless hours of music were recorded, re-recorded, cut up, edited, pieced together and constructed into never-completed tracks.

Wilson brought in songwriter Van Dyke Parks to compose lyrics for his new music, which spiraled wildly from one genre – doo-wop! – to another – psychedelic folk! Pieces were based on ages-old classical works and standards. Moldy period movements like ragtime, vaudeville and barbershop were resurrected. And Wilson swathed it all in an avant-garde coating that sounded like nothing at all in pop music – certainly not anything the Beatles or Spector were making.

But Wilson's demands – rooted in a perfectionism to get the exact sounds in his head onto tape – frustrated his fellow bandmates, who weren't used to spending months on end recording a single album. Plus, they weren't really sure what Wilson – whose experiments with LSD and other drugs were influencing his work – was up to with Smile. It was just too weird and abstract for them to wrap their heads around. Songs like "Do You Like Worms?" and "Vega-Tables" (in which Wilson sings about his favorite vegetables) were a long way away from "Surfin' USA" and "Fun, Fun, Fun."

By May 1967, 15 months after "Good Vibrations" was recorded, everyone had had enough of the endless sessions, Wilson's obsessive behavior and the fact that all of that work didn't seem to be in any more releasable shape than it was a year or six months ago. Wilson retreated, the other members of the band regrouped and the plug was pulled on Smile.

And its legend grew.

Over the years, everyone from the Beach Boys to bootleggers to Wilson himself had tried to reconstruct and make sense of the work. The group released Smiley Smile in September 1967, using some of the tracks and fragments from Wilson's discarded mess. A handful of songs surfaced on the Good Vibrations:Thirty Years of the Beach Boys box set in 1993. In 2004, Wilson even recorded a new version of Smile with a new band.

Finally, in 2011, the five-disc Smile Sessions became the final word on the subject.

Comprised of various sessions, takes, studio chatter and instrumental passages, the set constructs the basis of what Smile may have sounded like had it come out in 1967 (the structure was based on 1997's The Pet Sounds Sessions box, which documented, over four discs, the making of that classic record).

Listen to the Beach Boys' 'Heroes and Villains' From 'The Smile Sessions'

The first disc is made up of 19 tracks – including "Heroes and Villains," "Surf's Up" and a different version of "Good Vibrations" – that come pretty close to what Wilson has later said he envisioned for the record (even though cover art had been drawn up at the time that showed the album consisting of a dozen songs, which is more in step with the era).

Piece by piece, The Smile Sessions chronicled the making of the abandoned album. The key tracks are all on the first disc (which was also released individually from the box), but there's no denying the historical significance of the entire collection. The music isn't always easy to get into, and the oblique nature of many of the songs would certainly have kept them off the radio at the time (and no doubt confused or scared many of the group's fans). But it mattered as much in 2011 as it would have in 1967.

The original Smile, however it would have sounded, is a great, if flawed, work that The Smile Sessions finally tried to make sense of, 44 years later. And it's one that was immediately lauded following its release (it won a well-deserved Best Historical Album Grammy in 2013). It's also vindication for Wilson, whose mental issues were exacerbated by his inability to finish Smile at the time, and sidelined him for years.

Back then, he saw the LP as a forceful push against pop music's boundaries. The record, like much of Wilson's concepts, was ahead of its time. Even in 2011, Smile sounded displaced, a product of neither then nor now. Pop music was never ready for this, and it still may never catch up.

The Top 100 Rock Albums of the '60s

More From Ultimate Classic Rock