The shocking death of Thomas Becket is brought to life in an opera

The courtier is talented and loyal. Promoted to chancellor, he ruthlessly implements his master's will. But, when given a top religious job as well, he unexpectedly resigns his secular one and changes overnight from hedonist to ascetic: now he's the people's hero and the scourge of the rich. His master explodes in fury; the two men, once bosom pals, effect a partial reconciliation, but the former courtier's new-found principles provoke another row. The enraged master berates his henchmen for letting this upstart run riot: they confront the upstart at his workplace, and kill him.

The eight centuries separating Henry II and Thomas Becket from us don't in any way vitiate the story. Though contemporary accounts exist - Becket's murder and quick canonisation put Canterbury on everybody's map - most modern Britons know the story through one celebrated but inaccurate quote: "Who will rid me of this turbulent priest?"; and possibly, also, through two pieces of theatre: Jean Anouilh's Becket, ou l'amour de Dieu - which gave the tale French Resistance overtones - and TS Eliot's dirge-like Murder in the Cathedral.

But this general ignorance - and in particular the lack of information about a probable friendship between Becket and Henry's wife, Eleanor of Aquitaine - is seen by the performance-poet Philip Wells as a golden opportunity for the libretto he has written on the subject. "I'm not a historian," he says. "I wanted to go for the gaps in people's knowledge. And I have a hunch about Becket. Those gaps gave me the licence to imagine." The opera in question, King, has been composed by Stephen Barlow and will be performed by a cast including scores of local schoolchildren on the very spot where the original events took place.

It all began at a dinner party, where Barlow and Wells got talking about music theatre. A year later, they met at another dinner party and picked up where they'd left off. Wells said he'd seldom seen music-theatre work properly; Barlow asked him if he'd like to have a go. Then they discovered a mutual interest in Canterbury, where Wells had studied theology and Barlow had sung in the cathedral choir. Meanwhile, Barlow had been thinking about performance spaces: "I suddenly thought, 'how wonderful to produce a piece whose inspiration came out of a building'. So that you would be listening to the building as well as to the performers - rather than being in an anaesthetised concert hall or theatre, which in themselves tell you absolutely nothing."

It was but a short step to Becket. Wells's hunch was about the archbishop's motivation: "He was an essentially spiritual man, much more spiritual than the bishops who were his enemies. He was very close to his mother, who died when he was young. He wept easily, he had feminine aspects, and he had inner certainty. The more I read about him, the more fascinated I became."

When Becket's body was undressed after the murder, he was found to be wearing a hair-shirt, which impressed Wells even more. "That proved he was serious. How extraordinary not to have bragged about it when he was alive." Wasn't he just a masochist? "No. It's simply that privation is not a concept familiar to us now, it's very unfashionable. He was on a mission of unceasing prayer, and he had to stay awake. He was totally focused on what God wanted him to do. His story was one of the great tales of psychological transformation." Becket was, in his own day, compared to Saint Paul, in that he had a similarly dramatic conversion. "In our secular society we're not allowed to talk about these things," Wells adds. "We've lost track of the idea of being holy. And the feminine sensibility has been pushed out of spirituality. I hope that will now come through."

Barlow and Wells decided to focus on the relationship between Henry and his archbishop: Barlow instinctively sympathised with the former, and Wells clearly with the latter. The whole plot turns on the question of integrity, they think. Barlow: "Becket had never before been asked to tread over the line between doing something you feel OK about and something you don't." Wells: "It was like Twelve Angry Men. He stood up to the rest, and got hacked to pieces for it. This is a story about the workings of God through man."

The cathedral itself dictated the evolution of the work. Wells says he started out with no clear form: "I just wanted to listen to what the cathedral said to me. I did an all-night solo vigil - I was locked in alone, just wandered around, and prayed a bit. That experience changed me."

The pair then went on a pilgrimage, walking to Canterbury from Cheapside. "We felt like idiotic hippies, we just couldn't help smiling." Romantic? "Yes, but I'm a punk romantic," says Wells. "We have to break things before we make things."

A key part of this project is the sub-opera devised over the last year by children from Kent schools. "We listened to their angle," says Wells. "And they surprised us by the quality of their understanding - the story inspired them. They were fascinated by the idea of sacrifice, whether it was Jesus, or suicide-bombers, or just standing up to their mates when they disagreed with them." At key moments in the action, the children will speak their own words in unison.

The cathedral will look very much as it did at the martyrdom. As that happened to one side at the top of the nave, it will happen there again. So people won't see it? "No", says Barlow, "but they will be aware of it as people were in 1170 - the beggars and passers-by in the nave would have heard it in the most horrific way, and would have seen the monks running through the cathedral, and seen the body carried out."

This is apparently how they will begin the opera: the audience will come in to the sound of vespers, and will hear music which is as close as possible to the music the original spectators would have heard.

The key scene in the opera will be Becket praying, and there they will have the protagonist's thoughts accompanied by a tabla.

What sort of afterlife do they want for this opera? It could run with other school projects, says Barlow, hopefully. "And it could be done in other churches. I would love it to be done in Durham or Salisbury."

'King', Canterbury Cathedral, 28 to 30 April ( www.kingopera.com)