The venue was a smart London house in the early 1990s. David Cameron, then a rising star of the Conservative Research Department, was playing poker with friends.
The group of young, mostly Tory, Oxford graduates had been draining wine and whisky through the night. Then a cannabis joint began circulating around the card table.
When it reached the future Tory leader, he waved the joint away. It was an ostentatious refusal, put down to the limiting requirements of political ambition by his fellow players.
By then, of course, Cameron had learned how careful he needed to be at such gatherings. Having been busted at Eton for smoking cannabis - when named by a fellow smoker - Cameron wasn't about to wreck his starry political career by indulging in illegal drugs in company.
More than a decade later, as Cameron stood on the brink of the Conservative leadership in October 2005, the issue of drugs threatened to do just that.
At a fringe meeting at the party's annual conference in Blackpool he was asked whether he had taken drugs while he was a student at Oxford in the 1980s. Cameron then uttered six words that sparked an extraordinary media witch-hunt: "I had a normal university experience."
Pressed as to what that meant, he said: "There were things that I did then that I don't think I should talk about now that I'm a politician."
But although Cameron won the leadership contest, the issue of whether he took drugs and, if so, what and when has lurked in the shadows ever since.
What drugs exactly did he take at Oxford? Did he take cocaine when he was a public relations executive at Carlton Television?
His upper-class background fuels the innuendo. His was a world, after all, of Notting Hill aristocracy where cocaine is hardly unknown. Through his younger sister, Clare, he knew Jade Jagger. Jagger was expelled from her school for sneaking out to see Josh Astor, who was t himself expelled from Eton for smoking cannabis during the 1982 purge. Astor was later jailed for using cocaine. (At Oxford, Cameron once took Jade and Clare punting - an outing that her father Mick Jagger, mishearing it as "hunting", phoned to complain about the next day. "You know I don't approve of blood sports," drawled the Rolling Stone.)
But while some of his wider circle may charitably be described as louche, Cameron was either very discreet or abstained after his own brush with expulsion at Eton. One Oxford friend recalls that while others around him were experimenting with speed, the most Cameron had was "occasionally a joint or something".
Fine wines and beer rather than herbs or powders were Cameron's poisons. And even in legal intoxication, Cameron avoided the worst excesses of some of his peers.
His membership of the Bullingdon Club, an elitist Oxford dining society, is now something he regrets. The "hooray" images of lordly students in £1,000 double-breasted tail coats drunkenly smashing up furniture hardly fits with his message of "modern compassionate Conservatism".
One night, members of the "Buller" threw a pot plant through a restaurant window. The police were called and arrests made.
"The Etonians" (as one present described them) made a run for it. One who witnessed the event remembers the innocent making a quick getaway. "Boris Johnson turned out to be remarkably nippy for a cruiserweight, his bulky torso seen disappearing over Magdalen Bridge on a pair of skinny legs."
A passing taxi-driver saw two of the students and called out, "Hop in, they've just arrested your mates over there", and they drove off into the night. Typically, Cameron ("tired and in need of rest", according to one eyewitness) had gone to bed before the incident, all too aware that trouble was brewing.
On another occasion, his fellow Bullingdon Club members trashed his room at Brasenose College where he was studying PPE (philosophy, politics and economics). Cameron, called before the college Dean, paid a fine rather than name the vandals.
University contemporaries Giles Andreae and Dominic Loehnis both say they have never seen Cameron "out of control" drunk. "He would have got off his face at the Bullingdon," says a close friend, "but all that vomiting and so on would not have been him at all."
Another friend, who is no stranger to disciplinary procedures, says: "All that stuff with people being sick over each other just wasn't his thing at all. He was a responsible sort of person. Without being square, what flicked his switch was wit and repartee. He just wasn't the sort to get roaring drunk and destroy the fittings - he wasn't nearly wild enough. If he was in company when people were doing that sort of thing, he would worry and say, 'Oh, don't do that'."
Even as a Carlton Television executive Cameron seems to have been careful not to indulge. His refreshment of choice when entertaining journalists at television festivals was weak lager - and that in moderate quantities.
All Cameron's discretion and moderation couldn't hide the one hard fact of his drugs history, however. His school disciplinary record was a secret that was almost revealed at least once before Cameron even became an MP. In 1994, his former headmaster, Eric Anderson, attended a party at the London home of Michael and Sandra Howard.
"What on earth are you doing here?" Howard recalls Anderson asking Cameron. "Good God!" he exclaimed on being told that he was the Home Secretary's special adviser. Howard also remembers Anderson telling Cameron that no boy of his year gave him more trouble.
The then Home Secretary took it as a joke, which it surely was, but for Cameron it must have been an uncomfortable moment.
(Cameron has told friends that although he supported most of Howard's tough legislative programme, he was uncomfortable with the Home Secretary's crackdown on illegal raves - not least because his girlfriend, Samantha Sheffield, who he was later to marry, attended the events.)
There was another close shave when, in a 2000 battle between the Tory modernisers and traditionalists, seven members of the Shadow Cabinet confessed to having smoked cannabis.
The current Tory party chairman, Francis Maude, was among those who admitted smoking cannabis when young, in a rebellion against the party's "zero tolerance" policy towards the use of soft drugs. Others still on the Tory front bench who admitted trying the drug were David Willetts, the current shadow Education Secretary and Oliver Letwin, who heads Cameron's policy development.
William Hague, then the party's leader and now shadow Foreign Secretary, joined Ann Widdecombe in denying that they had ever smoked cannabis.
Selected as a candidate for the safe Tory seat of Witney, Cameron - together with all such candidates - was instructed to answer drugs questions honestly. In the event, the questions never came and he entered parliament at the general election of June 2001 unscathed.
He hardly hid from the issue once safely elected, though. He threw himself into the work of the Home Affairs Select Committee and pressed for it to look at the issue of drugs laws.
A libertarian by inclination, Cameron had been sceptical of the effectiveness of laws on drug and alcohol use since his days as Howard's Home Office special adviser. He supported the downgrading of the cannabis classification from "B" to "C" and even suggested that the Government consider downgrading that of ecstasy.
The current party leader was also in favour of heroin "shooting galleries", where addicts could more safely inject their drugs. His support for such harm-reduction measures raised eyebrows among his party colleagues when the select committee's report was published in 2002.
It was so controversial that another cross-party member of the committee, Angela Watkinson, MP for Upminster and now a whip, refused to endorse its conclusions.
In urging improvements to drugs rehabilitation programmes Cameron was continuing a campaign waged by the man who first helped inspire him to become an MP. His late godfather, Tim Rathbone, who had been the Tory member for Lewes, was an impassioned supporter of better treatment for addicts. (When Cameron worked briefly in Rathbone's office after Eton he was set to work researching the issue). But Rathbone, the most liberal of Tories, was out of step out with his party and was eventually expelled by William Hague for urging members to vote for a breakaway pro-Euro Tory party.
How will Conservative grass-roots members react to the news that their leader has used illegal drugs? Most will probably have assumed he had smoked cannabis and be relieved that no evidence has emerged of abuse with harder drugs.
But Cameron knows that today's revelation will lead to a re-run of the media frenzy he endured in October 2005. Sixteen months ago, he issued a partial denial suggesting that he had not taken drugs since he entered politics. Later, he said he believed that "law-makers shouldn't be law-breakers".
The frenzy died down when Cameron said that a "close relative" of his had received treatment for drug addiction: "Someone very close in my family has had a dreadful problem with drugs. They have been through it, been through rehabilitation, and I'm incredibly proud of them."
But his distinction between what he did during his political career and his private life before will be sorely tested in the days and the weeks to come.
Friends say that his refusal to answer the drugs question during the leadership campaign was motivated by fear that his school disciplinary record would be unearthed. It led to persistent suggestions that he had taken class "A" drugs, but Cameron believed it was more important not to be caught lying if the Eton incident came to light than to close down damaging but unsubstantiated speculation about harder drugs.
Now that the "Eton bust" is out in the open, the Tory leader will be asked what else has remained secret about his drugs past. He will do all that he can not to give into the pressure to say more.
Gordon Brown, the man who he is soon likely to face across the dispatch box, has issued a total denial of all drug-taking. After the 2000 Tory admission, Brown's office issued this statement: "He answered the question in public and the answer is no, no to cannabis and all other illegal drugs."
David Cameron smoked cannabis - and inhaled - but, so far, has not exhaled any "drugs lies". He is betting that the British public will forgive the first - but would not forgive the second.
CAMERON ON DRUGS
"People close to me have had their lives ruined by drugs, and I want us to tackle the problem properly"
"I'm allowed to have had a private life before politics, in which we do things that we should not and make mistakes"
BBC1's 'Question Time', 2005
"There's a suspicion that, because of my age and background, I have a tolerant attitude towards drugs. Nothing could be further from the truth"
'Daily Mail', 2005
"It's against the law and I've seen at quite close hand what happens when people do abuse drugs, and the nightmare their life can become"
On cannabis on BBC 1's 'Sunday AM', 2005
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