When it came out on 1 June 1967 it was unlike anything that had come before it. For many, Sgt Pepper is The Beatles at their best.
Half a century on, it remains one of the most influential albums of all time, as Beatles author Steve Turner explains: "It was artistically aspirational at a time when most of their contemporaries were still playing beat music of some kind or another.
"They had these great ambitions, Paul was starting to go to the theatre, George was listening to Indian music, and all these new interests came in and affected the album."
Fans around the world clambered to get hold of a copy, Peter Blake's iconic artwork on the cover meant it immediately stood out. But the real revolution was how it sounded.
It was one of the very first rock and roll concept albums - 13 tracks about an imaginary world where a fictional concert is taking place led by a character called Sgt Pepper.
It drew listeners in, using studio techniques that had never been tried before.
The band spent hundreds of hours perfecting the recording with the legendary "fifth Beatle" Sir George Martin, as his music producer son Giles can testify.
"They would stay in late at night, in the classic British fashion, EMI locked the fridge, so they broke the lock on the fridge in order to get milk for the tea."
During the recording, things occasionally got a little stronger than tea.
"I think they were recording Lucy in the Sky with Diamonds," Giles laughs. "John had disappeared and he was up on the roof looking up at the stars and I think there were...other influences involved."
The album has sold more than 5.1 million copies in the UK and it seems on course to get to number one this week with its 50th birthday reissue.
In Liverpool, 13 art projects have been commissioned to mark each track on the album. Turner prize-winning artist Jeremy Deller's work will examine manager Brian Epstein's contribution.
"I believe, without him the band wouldn't have existed as we know it," he says.
"[He is] an incredibly important character in British social history."
Few albums have had the same longevity or passed through successive generations in quite the same way.
Since September, it's even become a compulsory part of the exam board AQA's GCSE music syllabus.
Geoffrey Higgins, Director Of Music at St Augustine's Priory school in Ealing, has been teaching it.
"The beauty with Sgt Pepper is that the students can learn so many different musical features.
"On one hand they can access the standard pop traditions, like standard chord progressions, on the other hand they can access the more unusual aspects of the music which the album has become so famous for.
"You have the psychedelic influence with Lucy in the Sky with Diamonds, unusual chord progressions, the influence of Indian classical music and even the technological advancements that the Abbey Road engineers were making at the time."
Student Molly Agnew, 14, says she now sees its appeal.
"Actually it's very similar to a lot of the music you would hear now, with Harry Styles coming out with a very Beatles theme at the moment.
"If people really listened to them I think they would learn it's actually very similar, nothing's really changed."