OF all the momentous events that helped build the legend of the wartime French Resistance over the years, one episode outstrips the rest for its combination of tragedy, mystery and high-octane drama.

In France, they refer to it simply as "the raid on the house in Caluire." To the rest of the world, it is the story of how the Gestapo finally laid hands on Jean Moulin.

Jean Moulin was the former préfet (governor of a département) who in January 1942 was sent by General de Gaulle to organise the anti-German underground. For a year and a half he travelled incognito around occupied France, using the pseudonyms Rex then Max. Under his aegis, the Resistance grew from a patchwork of hostile grouplets into a unified structure with fighting potential.

But the end came on June 21, 1943 at a doctor's house in Caluire, an affluent suburb of the southeastern city of Lyon. A clandestine meeting of Resistance leaders had been called to make arrangements following the arrest of a senior colleague. But someone had tipped off the Gestapo and its notorious local chief Klaus Barbie. Moulin was arrested with seven others. After prolonged torture, he died on a train to Berlin.

Extraordinarily some 70 years later, the man who walked with Jean Moulin across Lyon to take part in that ill-fated meeting – who stood next to him in the doctor's waiting-room as they were handcuffed by Barbie's men – is still alive to tell the tale. Raymond Aubrac is France's last survivor from the senior ranks of the Resistance. He is 97 and slightly stooped, but otherwise hale and more than happy to relive those extraordinary times.

"When you are living your life on the run, as we were, you are constantly worrying about being arrested," he says. "So when the Gestapo burst into the house, it was a shock but not a surprise. The first reaction is to focus on the false name on your identity card. You know you are going to have to answer questions, and it is not going to be easy. There were two schools of thought when it came to interrogation. The cleverer, stronger types had a false story ready, full of details, which they built up to send the Gestapo down the wrong track. But if you have a bad memory, like I did, it is better to say nothing – just give your false identity and keep silent."

Aubrac, needless to say, was not the name that Aubrac gave to Barbie that day. At this stage of the war he was going under the name Claude Ermelin. A few months earlier, as François Vallet, he had been arrested by French police and then handed for a short time to the Gestapo before being released. In real life, he was Raymond Samuels – a Jewish name he obviously preferred to keep secret.

Born in eastern France in 1914, Samuels/Aubrac had studied engineering in Paris and in 1937 spent a year at MIT in Boston – whence came his precise English. He was also involved in left-wing politics – a supporter of the Communist Party if never a member. After the outbreak of war he married fellow left-winger Lucie Bernard; he saw military service during the battle for France before being taken prisoner and then escaping; and then, in late 1940, the couple settled in Lyon.

"I never 'joined' the Resistance because at the beginning there was nothing to join," he says. "It started off with us buying boxes of chalk and writing graffiti on walls. Then we progressed to writing tracts and putting them through people's letter-boxes. And then the third stage was our newspaper, Libération. It's when you have an underground press that you can first talk of an organisation – because you need a proper structure for it to work.

"What was my motivation? My country was at war! People forget that France lost 100,000 men killed in the fighting of 1940. And now Germany was taking 60% of our agriculture to feed its people; 70% of our industry. And all with the help of our so-called Vichy government. It was intolerable."

By mid-1942, Aubrac had become an important player in Moulin's nascent Resistance. The two men first met within days of Moulin's arrival by parachute in Provence, and Aubrac was put on the organising committee of the so-called "Secret Army". This was the paramilitary body that brought together the fighting units of the various underground groups. In June 1943 it was the sudden arrest of the Secret Army's leader – Aubrac's superior General Charles Delestraint – that triggered the Caluire conference.

By that time Aubrac had met Moulin on several occasions and, like everyone else, he had fallen under his spell. "He is very difficult to describe, because in physical appearance he was very normal – except perhaps his eyes," says Aubrac today.

"But it was his way of discussing matters that was so interesting. Never once did he use the way of authority. He had real power – over money, over communications, over all the agents. And many in the Resistance could have seen him as an enemy. But he never forced his ideas on people. Instead he used a kind of Platonic discussion method, so that all views were aired.

"He was indeed a remarkable man. And do you know for the last 70 years, every time I find myself confronting a problem I always ask myself what Moulin would have advised me to do. That was the kind of person he was."

After Caluire, Aubrac saw Moulin only one more time. It was at the Montluc prison in Lyon, where they were taken after the arrests. "My cell was on the first floor. There were eye-holes in the doors which were meant for the guards, but we could also use them to look out. And the last time I saw Moulin, he was being carried down the stairs outside my cell by two SS men. He was in a very bad state. Only later did I learn that he was being taken to Paris, and from there on to Berlin. But he died on the way."

Aubrac's subsequent story is another chapter of courage and derring-do. Within weeks of his arrest, he was sentenced to death by a court in Paris. "But luckily they did not shoot me straightaway. That was standard practice. They would wait because they thought we could still be useful to them in some way." The delay gave Aubrac's wife, Lucie, time to form an escape plan.

How Lucie and her Resistance group sprung Aubrac from the clutches of the Nazis is today one of France's best-known stories from the war – as uplifting for the French as the Caluire episode is grim. Somehow Lucie managed to persuade the German commander that she was a) pregnant by the prisoner Aubrac (this was actually true) and b) unmarried to him. By feigning horror at the prospect of the child being born out of wedlock, she got the commander to agree to a pre-execution marriage.

And so on October 21, the convoy taking Aubrac back to Montluc jail from his "marriage" ceremony at police headquarters was attacked by a heavily-armed Resistance gang. Three Germans were killed and 14 prisoners excaped. A few months later Aubrac and Lucie were picked up by an RAF Lysander from a secret location north of Lyon and flown to London. The call-sign on the BBC confirming the rendezvous – Ils partiront dans l'ivresse (They will leave in a state of drunkenness) – was later the title of a book of Lucie's memoirs.

And there the story would happily end – with Aubrac going on to a successful post-war career as a minister and administrator, and Lucie embarking on her long life as activist and educator. She died in 2007. But there is a coda to the story. Because nearly 70 years after the arrests at Caluire, historians are still at loggerheads over the detail of what occurred – and above all over the central mystery of the affair: who was the traitor?

For Aubrac, the question has long been settled. Like most people, he believes the guilty man was René Hardy – a middle-ranking Resistance figure who had been arrested by the Gestapo and then released a short time before Caluire. Originally Hardy had not been invited to attend the meeting, and after the arrests he was the only prisoner not put in handcuffs. As they were making their way to the Gestapo vehicles, he made a dash for it and managed to escape – albeit with a gunshot wound.

After the war Hardy was twice put on trial, but on each occasion he was acquitted for lack of evidence and he went to his grave in 1985 protesting his innocence. And so other theories have been given room to grow – and the most notorious of them fingers none other than Aubrac himself.

Historians have found a number of contradictions in the various accounts given by Lucie and Raymond Aubrac over the years. For example, it only became known long after the events that not just Hardy, but Aubrac too had been questioned by the Gestapo and then released in the weeks preceding Caluire. And why was he the only one of the prisoners not taken to Paris after the arrests, thus making possible his escape? Above all, there is the fact that he and Moulin arrived on foot 45 minutes late for the Caluire conference. Was it only coincidence that the Gestapo also arrived late – just five minutes after he and Moulin?

Today none of this fazes Aubrac, because he has answered the charges a hundred times – most notably following the publication of a J'accuse-style book in 1997. A committee of historians on that occasion revisited the evidence and concluded that though the various Aubrac accounts were certainly inconsistent and therefore suspicious, the charge of betrayal was impossible to sustain. As more than one pointed out, the first allegations after the war that Aubrac was the traitor were made by Barbie, who was not exactly disinterested.

The most ringing response came from Daniel Cordier, Moulin's former secretary and today, along with Aubrac, the last person alive who knew him well.

Cordier says: "People seem to forget when they ask Resistance members to explain their actions, they were not elected to the job, nor were they state-appointed officials - If there were acts of imprudence, mistakes, betrayals – whose fault is that? We all had these positions of immense responsibility – for which we were eminently unqualified – for one simple reason: there was no-one else."