AN OLD man sits in his chair and is approached by his great grandson.

"Papa, you were a sports writer," the boy says, "did you ever do anything good, or important?" The old man does not pause. He says "Lance Armstrong."

The scene cuts and then there is a fade to a street in Liege, Belgium, David Walsh, that great grandfather to be, is pulling his case behind him. He has toothache. He has 2000 words to write about the Tour de France. He has been abandoned by fellow pressmen who have said they cannot give him a lift to the next stage.

The events that stretch between the lonely promenade in Belgium and the family scene in blissful domesticity will now be the subject matter for three films commissioned by Universal, Paramount and Warner. Two documentaries are also in production.

The downfall of Lance Armstrong, seven-time Tour de France champion, is more than a sports story. It is an almost overblown drama of obsession, bullying, cheating, deception, life-threatening illness, intrigue, baffling strength and deeply human weakness. And drugs, of course.

The Glasgow Science Centre on a bright Tuesday night is an appropriate setting for talk of haematocrit, EPO, laboratory testing and saline drips. Walsh, the Sunday Times journalist who spent 13 years pursuing Armstrong, stands in front of more than 300 people and tells the story of how the American was finally exposed.

It is a precis of Walsh's book, Seven Deadly Sins, about how the cyclist used drugs, most spectacularly EPO, to fuel his surge to the top of the sport.

The night is sprinkled with extraordinary scenes: there is Armstrong's confession of illegal drug use as he lies in a hospital bed suffering from cancer, there is the cyclist's bullying of those who were once his friends, there is, too, the burning hatred of Walsh and an almost exquisitely painful scene where the writer's dead son is invoked in a moment of crass, awful insensitivity by Armstrong.

For this is a story, too, of obsession on the part of Walsh. The death of his son, then 12, in a bike accident outside their home in Ireland had an obviously profound effect on the family but his boy's insistence on asking the awkward question was inherited by his father.

John Walsh, as a primary school pupil, had posed a simple but unanswerable inquiry to his teacher. On hearing that Mary and Joseph had received gold, frankincense and myrrh from three wise men, John was curious about why the parents of Jesus subsequently lived a poor, humble life. "What happened to the gold?" he asked.

His father, after being hugely impressed by Lance Armstrong as a novice cyclist, took this interrogation technique into his subsequent pursuit of the American. Walsh simply could not believe the narrative that was being spouted about the multiple champion. He and a few others, most notably two journalists, Paul Kimmage and Pierre Ballester, refused to be cheerleaders.

Walsh was thus sued, vilified by Armstrong as a "troll" and shunned by journalists, who feared correctly that any proximity to the Irishman would distance them from the camp of the champion.

The journalist stands in front of the Glasgow crowd and relates all this on a day when the Godolphin horse training empire has been accused of using anabolic steroids, when the trial of Dr Eufemiano Fuentes, who has implicated tennis players and footballers in drug-taking, rumbles on and the news is seeping out that Armstrong may be sued by the US government. Walsh believes some footballers and tennis players are drugs cheats and is scathing over the paucity of efforts to catch them.

However, his specialist subject in the book is journalism. Walsh has shown the power of awkward questions complemented by an undying obsession to trap the most brazen example of drug-taking in sport.

"I have said all along that it was great fun doing it," says the journalist before the Tour de Walsh begins in Glasgow. "But it was not great fun being in the salle de presse because one had the impression that most of the people there were made uncomfortable by my presence. I was the guy who reminded them of the case against Armstrong and they did not want to be reminded of that."

This obdurate persistence is just one of the qualities that make the Armstrong story so human. "This is about more than cycling. The bike is just a prop in the Armstrong story. This is a unique tale and that is why Hollywood has commissioned films on it."

Walsh now moves on to cover his first Tour de France in nine years. He will travel in considerable comfort. There will be no pulling a case along a street, no desperate cadging of a lift. Instead, he will travel in the Team Sky bus as David Brailsford, the general manager, has given the journalist full access to operations and to competitors. Walsh believes that Bradley Wiggins of Team Sky was clean when he won last year's tour but he welcomes the chance to peek in the corners.

Howver, the Armstrong chapter continues to spin as his one-time paymasters sue and criminal legal proceedings may ensue. The films will almost certainly be in cinemas before the courts clear the detritus of one of the greatest sporting scandals.

Walsh is convinced he knows the heroines of any script. Emma O'Reilly, the masseuse, who admitted fetching drugs for Armstrong spoke out and was labelled a whore. Betsy Andreu, wife of cyclist Frankie, testified to hearing Armstrong admit illegal drug use to a doctor and was ostracised and criticised. "Both of them deserve far more praise than me," says Walsh. "I was paid to do this. A cheque was going into my account from my newspaper every month."

The downfall of Armstrong was confirmed when the International Cycling Union stripped the cyclist of his seven Tour de France titles. This occurred on October 22, 2012. This would have been the 30th birthday of John Walsh, the awkward questioner of the Nativity story and the fatal victim of a bicycling accident some 18 years earlier.

"Betsy Andreu phoned on that day and told me the news about Lance was John's little birthday gift to me," says Walsh. It would make the perfect ending to a film. However, Walsh adds: "I do not know if I can quite believe that." It is the instinctive verdict of the inveterate sceptic in tragedy and in triumph.