On 29 August 1966, the Beatles closed their set at the Candlestick Park baseball stadium in San Francisco with “Long Tall Sally”, an old Little Richard number that had been part of their repertoire from the very start. “See you again next year,” said John as they left the stage. The group then clambered into an armoured car and were driven away. It was to be their last proper concert.

Their American tour had been exhausting, sporadically frightening, and unrewarding. By this stage their delight in their own fame had worn off. They were fed up with all the hassle of touring, and tired of the way the screaming continued to drown out the music, so that even they were unable to hear it. Having been shepherded into an empty, windowless truck after a particularly miserable show in a rainy St Louis, Paul said to the others: “I really fucking agree with you. I’ve fucking had it up to here too.”

“We’ve been telling you for weeks!” came the reply.

On their flight back to England, George told press officer Tony Barrow: “That’s it. I’m not a Beatle any more.” Like the others, but perhaps more so, after 1,400 shows he was sick to death of playing live: at the age of 23, he had had enough.

For the first time in years, the four of them were able to take a break from being Beatles. With three months free, they could do what they liked. Ringo chose to relax at home with his wife and new baby. John went to Europe to play Private Gripweed in Richard Lester’s film How I Won the War. George flew to Bombay to study yoga and to be taught to play the sitar by Ravi Shankar. This left Paul to his own devices.

For a while, he enjoyed himself in London, immersing himself in the counter-culture, as the avant garde was then briefly known. By now he was one of the most famous men in the world. Even as a member of an audience, or a visitor to a gallery, he was always the centre of attention. So at the beginning of November he decided to conduct a little experiment: what would it be like to be normal?

Paul’s pursuit of anonymity had its showy aspect. It was almost as if he were acting not acting, or being ostentatiously incognito. His first stop was a company called Wig Creations, who dutifully measured his upper lip and gauged the exact colour of his hair in order to produce a false moustache, and provided him with two pairs of fake spectacles fitted with clear lenses. By slicking his hair back with Vaseline and donning a long overcoat, Paul found he could wander around unrecognised.

'I was a lonely little poet on the road,' said Paul. But few poets have cruised round Europe in a brand-new Aston Martin DB6

Suitably togged up, he set off on a solo motoring holiday in France. Looking back on that jaunt, he pictured himself as just another humble traveller. “I was a lonely little poet on the road,” he said, though few poets in history have been able to cruise around the Europe in a brand-new Aston Martin DB6.

Paul and his Aston Martin were transported to France by an exclusive air-ferry service operating from a little airport in Kent. On arrival he glued on his moustache, donned his overcoat and his glasses, jumped into his sports car, and headed off for Paris.

Once there, he wandered around by himself, filming bits and pieces, employing experimental techniques he had picked up from New Wave cinema and Andy Warhol. Sometimes he stayed in his hotel room writing a journal. At dinner he sat by himself, jotting down notes. He wanted to “retaste anonymity. Just sit on my own and think all sorts of artistic thoughts like, I’m on my own here, I could be writing a novel, easily. What about these characters here in this room?” It’s easy to forget that he was still only 24 years old, with his life ahead of him.

On reaching Bordeaux, he felt a hankering for the night life. Still in disguise, he turned up at a local discothèque, but was refused entry. “I looked like old jerko. ‘No, no, monsieur, non’ – you schmuck, we can’t let you in!” So he went back to his hotel and took off his scruffy overcoat, his moustache and his glasses. Then he returned to the disco, where he was welcomed with open arms.

By now, he had begun to recognise the shortcomings of anonymity. “It was kind of therapeutic, but I’d had enough. It was nice, because I remembered what it was like to not be famous, and it wasn’t necessarily any better than being famous. It made me remember why we all wanted to be famous; to get that thing.” Reflecting on his brief retreat from celebrity, he felt happier with his lot. Fame, success and money may well have their shortcomings, but they are still a lot better than their alternatives.

Facebook Twitter Pinterest The Beatles in Florida, 1964. Photograph: John Loengard/Time & Life Pictures/Getty Image

Before setting off on his jaunt, Paul had arranged to meet road manager Mal Evans under the Grosse Horloge in the centre of Bordeaux, reckoning that by then he would be in need of company. The two of them drove to Madrid, then down to Córdoba and Málaga, vaguely thinking they would hook up with John. But when Paul discovered that John had already left Spain he abandoned his plans, phoning Brian Epstein to send someone from London to drive his car home. Epstein also booked Paul and Evans flights to Rome, then on to Kenya, where they went on safari, with a night at Treetops Lodge in Aberdare National Park.

Paul flew from Nairobi to London on 19 November, ready to record the Beatles’ new album on the 24th. On the flight back with Evans, he was thinking about his recent assumption of another persona, and wondering if the Beatles might benefit from something similar. “With this alter-ego band, it won’t be us making all that sound, it won’t be the Beatles, it’ll be this other band, so we’ll be able to lose our identities in them.” But what would they call themselves? While he was wondering, their in-flight meals arrived, complete with packets marked “S” and “P”.

“What’s that mean?” asked Evans, before answering his own question: “Oh, salt and pepper.”

“Sergeant Pepper,” said Paul, without thinking.

•••

Even Beatles need dentists. To become the Fab Four, their teeth needed as much tweaking as their hair and clothes, perhaps more.

John Riley was the son of an upstanding south London police constable. He had studied cosmetic dentistry at Northwestern University dental school in Chicago before setting up a practice in Harley Street, London. He is said to have possessed a quality rare, and perhaps dangerous, in a dentist: charisma. “He was the sort of man that if he walked in the room, you’d feel his presence even before you saw him,” observed one of his clients. By the early 60s he had become one of the most fashionable dentists in town, much sought after by figures in showbusiness and the arts, including all four Beatles.

George’s teeth needed particular attention: photographs from the Cavern days reveal them as uneven and rickety. Through the latter part of 1963 and into 1964 he became such a frequent visitor to Riley’s clinic that the two men struck up a friendship of sorts, sometimes going out clubbing together. In February 1965 the Beatles even invited Riley to the Bahamas to keep them company while they were filming Help!. Yet Pattie Boyd never felt entirely comfortable in his presence, particularly when lying back on his dental chair with her mouth wide open. Pattie wondered if he might have taken advantage: “No matter what he was going to do in our mouths, he would give us intravenous valium. All of the Beatles went to him and we took it for granted that this was what happened – no one questioned it. We would go into a deep sleep and wake up not knowing what he had done. I watched him trying to revive George once by slapping his face. It was sinister – he could have been doing anything to us while we were out.”

Facebook Twitter Pinterest The Twist and Shout EP cover. Photograph: Fiona Adams/Redferns

In April 1965 Riley and his girlfriend Cyndy, whose job was to hire the bunny girls for the Playboy Club, invited John and Cynthia and George and Pattie over for dinner at his home in Bayswater. “We had a lovely meal, plenty to drink,” Pattie recalled.

As dinner came to an end, George and Pattie got up to leave, explaining that they were planning to see Klaus Voorman and his new band playing at the Pickwick Club, just off Leicester Square.

As Pattie remembers it, Cyndy then said: “You haven’t had any coffee yet. I’ve made it – and it’s delicious.” So they sat down and drank their coffee. Then John made another move to leave, explaining that Klaus was due on soon.

“You can’t leave,” said John Riley. “What are you talking about?” “You’ve just had LSD.”

“No we haven’t.”

“Yes you have. It was in the coffee.”

According to Pattie, John was “absolutely furious”. He had read about this comparatively new drug in Playboy magazine. “How dare you fucking do this to us!”

In great waves, the LSD took effect. Cyndy thought time had stopped, and as if this weren’t bad enough, that they were all going to drown. “The Bismarck is sinking! The Bismarck is sinking!” she shouted, over and over again. Pattie felt strongly that she didn’t want to stay there: “I wondered if the dentist, who hadn’t had any coffee, had given it to us hoping the evening might end in an orgy.” George entertained similar suspicions.

The four guests insisted on leaving. Riley said they shouldn’t drive, and offered to drive them himself. They refused, and the four of them squeezed into Pattie’s Mini and set off. Pattie was convinced it was shrinking: “All the way the car felt smaller and smaller, and by the time we arrived we were completely out of it.”

They stumbled into the lift at the Pickwick Club, then became convinced that the little red light in it was a raging fire. When the doors opened they emerged screaming into the club. Riley had followed them in his car. He sat at their table, and turned into a pig.

Pattie was discombobulated: “People kept recognising George and coming up to him. They were moving in and out of focus, then looked like animals.” They left the Pickwick and walked to the Ad Lib, in Leicester Place: “On the way I remember trying to break a shop window.” As the lift doors opened there, they crawled out and bumped into Mick Jagger, Marianne Faithfull and Ringo. “John told them we’d been spiked. The effect of the drug was getting stronger and stronger, and we were all in hysterics and crazy. When we sat down, the table elongated.”

After heaven knows how long, they made their way home, with George driving “at no more than 10 miles an hour” all the way back to Esher. John couldn’t stop cracking jokes; LSD is the ideal drug for those addicted to puns, as it turns everything into something else, then back again.

The LSD took eight hours to wear off. Cynthia remembered the four of them sitting up for the rest of the night “as the walls moved, the plants talked, other people looked like ghouls and time stood still”. For her, it was “horrific, I hated the lack of control and not knowing what was going on or what would happen next”.

George and Pattie vowed never again to visit Riley. No one wants a groovy dentist, any more than they want a sybaritic bank manager or a butter-fingered brain surgeon. But while Pattie and Cynthia felt that the whole experience had been very frightening, George saw it as a revelation: “It was if I had never tasted, talked, seen, thought or heard properly before. For the first time in my whole life I wasn’t conscious of ego.” And John loved exactly what Cynthia loathed: the lack of control, the unexpected weirdness of everything. Within weeks of that first trip, he would be dropping acid every day.

•••

Facebook Twitter Pinterest Apple in June 1968; John Lennon is touching Derek Taylor’s head. Photograph: Jane Bown

On 4 December 1968, George circulated the following memo to all the Apple staff :

Hells Angels will be in London within the next week on the way to straighten out Czechoslovakia. There will be 12 in number complete with black leather jackets and motorcycles. They will undoubtedly arrive at Apple and I have heard that they might try to make full use of Apple’s facilities. They may look as if they are going to do you in but are very straight and do good things, so don’t fear them or uptight them. Try to assist them without neglecting your Apple business and without letting them take control of Savile Row.

A close reading of this breezy text might have sounded alarms. But the mood in Apple during that period was determinedly, almost compulsorily, free and easy.

The arrival of the Hells Angels was preceded by a call from Customs and Excise to the Beatles’ PR Derek Taylor. “Is this right? We’ve got two HarleyDavidsons that you’re going to pay the freight duty on?”

Obeying George’s order to assist, Taylor nodded it through, and also paid £250 for shipping. Presently, two Angels – Frisco Pete and Billy Tumbleweed – showed up at 3 Savile Row, along with the author Ken Kesey, a hippy called Spider, and 12 hangers-on: “zonked, wired”, in the words of Taylor’s assistant Richard DiLello, “smelly, stoned’ according to Brian Epstein’s former assistant Peter Brown.

DiLello witnessed this mixed bag of people “in all their splendor, sprawled across the reception lounge; laughing, smoking and reeking of patchouli oil. There were men, women and babes in arms; leather, suede, headbands, cowboy hats, bells, sleeping bags, backpacks, beads, mountain boots, sticks of incense, flutes and guitars.” Collectively they came to be known by the Apple staff as the California Pleasure Crew.

On their arrival at reception, Taylor came down to greet them, and proceeded to introduce them to everyone else as though they were visiting dignitaries, which, in a way, what they were. DiLello transcribed Taylor’s meandering greeting: “Well, you are here and so are we and this is Sally who has just joined us and that is Carol who has always been with us and Richard you know, and if you would like a cup of tea then a cup of tea it is, but if you would rather have a glass of beer or a bottle of wine or a scotch and coke or a gin and tonic or a vodka and lime, then that it is because it is all here and if it is not then we will come up with something, but have a seat or have a cigarette or have a joint, and I will be back in three minutes so please don’t go away because there is a lot to talk about and more to find out and stranger days to come.”

“Beer!” came their reply.

Their demanding attitude soon prompted Taylor to dispatch another memo to staff : Watch out, don’t let them take over. You have to keep doing what you’re doing, but just be nice to them. And don’t upset them because they could kill you.

The Hells Angels were still living at Apple on 23 December, the date set aside for the company’s first Christmas party. Everything had been meticulously planned. For instance, a turkey weighing 43lb – billed by the butcher as the largest in the UK – had been ordered, and invitations had been posted well in advance with the idea “that all of us at Apple will bring our children and those of us who have no children are invited to bring a couple unless they can arrange to have one of their own in the meantime. Immaculate conceptions will not be accepted”.

Do you dig us or don’t you? asked Hells Angel Spider. 'Yin and yang, heads and tails, yes and no,' replied George

By 9am on the big day Prudence and Primrose, Apple’s two cordon bleu cooks, were already hard at work. By 11am the press office was filled with journalists and assorted figures from the music business, slurping champagne. By 11.30am the Black Room was “swollen to standing-room-only proportions with hashish smokers puffing their brains out”. By noon, the music had been turned up loud, and the two different strands of party – drink and drugs – had merged into one.

The first gaggle of the anticipated 40 children arrived at 2.15pm.

Entertainers Ernest and April Castro launched into their splendid routine of magic, ventriloquism and barnyard imitations (“With a moo-moo here, and a moo-moo there …”) with their usual gusto. DiLello recalled hearing “eardrum-shattering squeals of delight” from the children.

Next, less than a month since their naked appearance on the cover of Two Virgins, John and Yoko entered the room dressed as Father and Mother Christmas. John’s childhood friend Pete Shotton felt that John was low on seasonal cheer: “He made for the most miserable-looking Santa I’d ever seen in my life.” Nonetheless, he managed to mutter “Ho ho ho” while he and Yoko handed out presents, the two of them assisted by the more Christmassy Mary Hopkin.

As the children unwrapped their parcels, the sound of barracking came from the back of the room. “Hey man! We want some food! GIVE US THE FUCKING FOOD, MAN!” It was the Hells Angel Frisco Pete, at his most peckish.

“They’re arranging it right now,” said John. “It should be out very soon.”

“What the fuck is goin’ on in this place!?! We wanna eat! What’s all this shit about HAVIN’ TO WAIT?”

At this point Alan Smith, a journalist with New Musical Express, stepped in, politely asking for a little consideration. Frisco Pete promptly punched him in the face.

Ten minutes later the doors to Neil Aspinall’s office were thrown open, revealing tables groaning with hors d’oeuvres, cold meat, jellied fish, salads, cheese and biscuits, cakes, fruit and boiled sweets. Resplendent in the centre of the table sat the largest turkey in the country, roasted to perfection by Prudence and Primrose.

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Hurtling past everybody else – staff and children, hippies, journalists, hangers-on – came Frisco Pete, who picked up the turkey, ripped off its left leg and started to gnaw on it. “It easily weighed 4lb, and more closely resembled a caveman’s hunting club than a turkey leg,” recalled DiLello.

By early January, George was starting to regret his open-house policy. Throughout the building things had been going missing, among them television sets, electric typewriters, adding machines, 300 copies of Two Virgins, a movie camera, three secretarial pay packets, half a dozen speaker cones from the studio, six fan heaters, an electric skillet, several cases of wine and all the lead from the roof. George now plotted to disinvite the California Pleasure Crew. At first he issued a directive to Taylor that he didn’t want them round any more. “What am I supposed to say?” Taylor asked.

In the end it was George, stubborn George, who finally gave the Hells Angels their marching orders. Once, when he had been in his teens, a man had come knocking at the door of his family home, trying to sell them something. George’s mother, Louise, instinctively knew what to do. She went to the upstairs bedroom with a tub of water and poured it over the poor man, yelling: “Go away!”

“I think George got his toughness from her …” reflected Paul years later. “She didn’t suffer fools gladly – and neither did George.”

If his recent embrace of hippy mysticism sometimes led George down the wrong path, there was still a more steely part of him that knew how to get back on track. So one evening he walked into the guest lounge of Apple and simply announced: “Hello everyone! Well, are you moving all of your stuff out of here tonight?” In turn, the California Pleasure Crew were struck mute.

Spider was the first to speak. “Hey, man. I just wanna ask you one question. Do you dig us or don’t you?”

“Yin and yang, heads and tails, yes and no,” replied George, enigmatically.

DiLello noticed an immediate change in the atmosphere. “The answer to that question completely fucked everyone’s mind. No one knew quite what to say or how to say it.”

Eventually, it was Spider who spoke up.

“All right, man, I can dig it. We’ll be outta here in 10 minutes.”

• One Two Three Four: The Beatles in Time by Craig Brown is published by 4th Estate (RRP £20). To order a copy go to guardianbookshop.com. Free UK p&p on all online orders over £15.