Two halves of one brain, JOHN LENNON and PAUL MCCARTNEY formed the most extraordinary and intimate musical partnership of the rock era. In this extract from a new book that probes their bond through the medium of their songs, Ian Leslie discovers how one of them, Sgt. Pepper's Getting Better, was a case of Paul reaching out to John, changed but also estranged by his new love... LSD.

From me to you: John Lennon and Paul McCartney get better and better, Paris, 1964.

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ON THE EVENING OF MARCH 21, 1967, three of The Beatles were at Abbey Road, recording backing vocals for a song called Getting Better. John, Paul and George were gathered around a microphone. After a few run-throughs, John took out a silver snuff box he kept his pills in and began poking around in it, searching for an upper to keep him going. Soon afterwards, he faltered and stopped in the middle of a line. He looked up to George Martin in the control room. “George, I’m not feeling too good,” he said. “I’m not focusing on me.”

Martin paused the session and took John up to the roof for some fresh air. The other Beatles stayed behind. But as McCartney and Harrison discussed what might be the matter with John, they figured out that he had probably taken a tab of LSD by accident – and that maybe standing on the top of a building wasn’t the best place for him. They rushed up the stairs, hoping that John did not decide to see if he could fly before they got there. As it turned out, he was OK. Still, work was halted for the night, and the band dispersed.

Paul and John stayed together. With the drug exerting its effects on his brain, John didn’t want to travel back to his home in Surrey. He and Paul headed for Paul’s house on Cavendish Avenue, a short drive from the studio. Once there, Paul decided he would take some LSD himself. Although he had tried acid for the first time in late 1965, that was with other friends. Now he wanted to “get with John”, as he later put it to Martin, who interpreted it to mean “to be with him in his misery and fear”. McCartney told Barry Miles: “I thought… maybe this is the moment. It’s been coming for a long time.”

That night, John and Paul did something that the two of them practised quite a few times during this period: they gazed intensely into each other’s eyes. They liked to put their faces close together and stare, unblinking, until they felt themselves dissolving into each other, almost obliterating any sense of themselves as distinct individuals. “There’s something disturbing about it,” recalled McCartney, much later, in his understated way. “You ask yourself, How do you come back from it? How do you then lead a normal life after that? And the answer is, you don’t.” The Beatles’ publicist and friend Derek Taylor recalled Paul enthusing about LSD: “We had this fantastic thing… Incredible, really, just looked into each other’s eyes… Like, just staring and then saying, ‘I know, man,’ and then laughing.”