AS the wine flowed at the Oxford Union debating society, the stage was set for what would soon become a broadcasting disaster.
The motion before the undergraduates had been "British politics is in the pocket of the media'' and in the exhilarated post-debate atmosphere, one dinner-jacketed journalistic insider, who had come from London to speak, could no longer contain his piece of political gossip.
Iain Overton, head of the small, non-profit Bureau of Investigative Journalism, said the following evening's Newsnight programme on BBC was going to expose a top Tory as an abuser of teenage boys at a north Wales care home.
According to one of those present, Michael Crick, former Newsnight journalist and now the Channel 4 News political editor, asked: "Do you mean McAlpine?"
"Well, you said it," Overton replied.
Over the next 24 hours, the incendiary tale went round media London. Overton poured petrol on the fire he had lit (very foolishly, as he has admitted to colleagues) by tweeting the next day: "We've got a Newsnight out tonight about a very senior political figure who is a paedophile."
He was later to tell friends that he in fact knew remarkably few details of the story.
It had been brought in to him by Angus Stickler, a Bureau reporter and a former BBC employee at Radio 4's investigative series File On 4. Under a routine devised to preserve the BBC's editorial control, Stickler was being seconded to Newsnight to work it up as a BBC project under the supervision of acting editor Liz Gibbon.
Newsnight's previous editor, Peter Rippon, was in effect under suspension, following the fiasco of the Jimmy Savile sex abuse story. Newsnight's own resources were further weakened because their reporter with the most hands-on investigative experience, Meirion Jones, was sidelined while he prepared his testimony to an external inquiry about why his own, completely accurate, Savile investigation had been suppressed.
Danger signs about the story were already flickering but it seems they were not noticed by those who should have briefed BBC director general George Entwistle -- who last night sensationally quit.
Crick called up an indignant Lord McAlpine, who lives with his wife in a former convent in Italy. He discovered that the sex-abuse allegation had never been put to the former Tory party treasurer by the BBC team.
Crick tweeted a public warning: "Senior political figure due to be accused tonight by the BBC of being paedophile denies allegations + tells me he'll issue libel writ agst BBC."
But it was too late. The programme went ahead, with an uncritical interview with Steve Messham, a victim of sexual abuse at the Wrexham care home in question. Messham said he had been taken in a car to the Crest Hotel in Wrexham and abused more than a dozen times by what Newsnight termed "a prominent Thatcher-era Tory figure". At first, it seemed that Newsnight had scored a remarkable journalistic coup, despite internet complaints that they had "bottled" naming the name.
British prime minister David Cameron and his home secretary, both visibly rattled, hastened to announce a pair of official inquiries into the allegation that the truth about a Tory paedophile ring had been suppressed. A hum of online chatter rose to a climax, as individuals in the know vied with each other to make clever, apparently artless 140-character remarks.
Sally Bercow, wife of the speaker of the House of Commons, tweeted: "Why is Lord McAlpine trending? *innocent face*." George Monbiot, the green activist who writes a column for the Guardian, wrote: "I looked up Lord #McAlpine on t'internet. It says the strangest things."
The 70-year-old McAlpine was driven from his home after coming under siege from journalists. But the profoundly embarrassing truth was that the story about McAlpine the paedophile was false from the start.
Not only was it a fairly straightforward case of mistaken identity, but this fact had been known about by veteran journalists for more than 15 years.
Messham's claims, however well intentioned, had already been carefully examined and rejected by the official Waterhouse inquiry in 1997. Simple journalistic checking would have revealed this.
McAlpine's cousin, Jimmie McAlpine, a prominent local businessman, was the one originally named by Wrexham inmates as the object of rumours. There was no evidence of actual sexual abuse in any event. These rather devastating facts are explicitly recorded in transcripts of the official Waterhouse inquiry.
On January 29, 1998, one of the lawyers at the inquiry, Sir Geoffrey Nice QC, referred to notes of a meeting between a social worker and a London-based Observer journalist, Dean Nelson, who had been investigating the home and its deputy head, Peter Howarth, who was eventually jailed for abusing the boys.
The journalist wrote that a number of boys had told him that Jimmie McAlpine, president of the Wrexham golf club, "was heavily involved with Howarth and was his frequent golfing partner".
Nelson explained to the inquiry: "The name McAlpine was one that cropped up regularly... but it was never a name you could pin down. I mean, there were lots of McAlpines and it wasn't something that I took seriously."
Nice said: "It's very frustrating from the tribunal's point of view that the name is mouthed and then you ask further questions and you've nothing to grasp at all... The furthest we have ever got is that somebody called 'Jimmie' was referred to."
Nelson replied: "I think Jimmie McAlpine had been involved in some benevolent charity work, sponsoring various children's playgrounds. Some people drew an innuendo on that, but I certainly didn't. I never heard any allegations... It was just one of the many, many, many rumours knocking around."
Years later, another former inmate, Keith Gregory, explained that inmates would be taken over to Gethyn Hall, Jimmie's Wrexham home, to till its grounds in "work parties", and that McAlpine would visit the care home in his expensive car.
Messham, who testified to the inquiry under the name 'Witness B', said he believed a McAlpine was one of those who had abused him. But he would not say who told him so or the man's first name.
The only concrete evidence he gave was that he believed the McAlpine in question was dead. Jimmie McAlpine had died in 1991. There was a flurry of journalistic interest at the time in 1998 in the possibility that the Tory treasurer was being referred to. Lord McAlpine was temporarily so hounded by the press that he was forced to move to New York for a while. But it rapidly became clear that the whole thing was a mistake.
The allegation went to sleep for years, until it was reheated by journalist Angus Stickler, and served up by Newsnight last week. As the witch-hunt mounted, ITV's This Morning host Philip Schofield even thrust at Mr Cameron a list of names, culled from the internet, of what he alleged were Tory paedophiles.
Eventually the Guardian came to the rescue of the former Tory treasurer on Friday, by finding and publishing the evidence that exonerated him.
Within 24 hours his accuser, Messham, had retracted and apologised for a case of mistaken identity, while the futures of the Bureau of Investigative Journalism, the Newsnight programme and the BBC boss himself have been all thrown into severe doubt.
Rarely can a journalistic balloon have been so totally empty -- and punctured so rapidly.