On 20 July 1944, a German colonel left a bomb in the Führer's office. It exploded, just missing its target, and the following day the officer was shot. With the daring plot now being made into a film - starring Tom Cruise - Berthold von Stauffenberg, son of the would-be assassin, tells Nigel Jones how his father's 'moral sacrifice' shattered his and his family's lives

Just before 1pm on a boiling hot day in 1944, a 10-year-old boy sat down to lunch at a grand country house in the hills near Stuttgart. An earnest young man, and the heir to one of Germany's most noble families, Berthold von Stauffenberg was in awe of the Nazi regime and talked excitedly about joining the Hitler Youth. But, 800 miles away, his father, Claus, had other plans. A colonel and a trusted member of the Führer's inner circle, he was, at that precise moment, trying to kill Hitler with a briefcase full of explosives. 'It was the closest anyone came to killing him,' says Berthold, now 74 and a fierce guardian of his father's legacy. 'He did what he did, and sacrificed his life, through sheer moral duty.'

Claus von Stauffenberg was a 'good German' amid a nation of demonised villains. An extraordinarily brave soldier and a charismatic leader, his attempt to kill Hitler on 20 July at the Nazi leader's eastern HQ - known as the 'Wolf's Lair' - will receive the Hollywood treatment in a new film, Valkyrie, starring Tom Cruise and Kenneth Branagh, being released next year.

Looming opposite me in a high-backed chair, Berthold, over 6ft and himself a retired major-general, seems to possess many of the same qualities. He has the natural authority and discipline of a military man and bears the burden of being his famous father's son with dignity and grace. When I meet his steady gaze, I think I understand how Claus persuaded so many co-conspirators to join his crusade and risk their lives.

Even so, Stauffenberg was an unlikely assassin. In Tunisia the year before the assassination attempt, Stauffenberg's jeep had been strafed by an American plane, leaving him with one eye and one hand - with just three fingers left on it. He had amazed his doctors by the speed of his recovery - learning to dress so dextrously using his teeth and three fingers that he joked that he did not know what he had ever used the other seven for - but the idea that this maimed staff officer could prime a bomb, then carry his heavy, suspiciously bulging briefcase into the dictator's presence and plant it under his nose, was unthinkable. Until he did it.

Murmuring that he had to make a phone call, Stauffenberg left Hitler's conference room and waited nearby, calmly smoking a cigarette and chatting with colleagues. At 12.42pm the bomb exploded. Without waiting to see the results of his handiwork, Stauffenberg leapt into his car, bluffed his way past two checkpoints, and boarded a plane for Berlin. Behind him, his fellow anti-Hitler conspirators were supposed to shut down the Wolf's Lair's communications network while their colleagues in Berlin launched 'Valkyrie', a plan devised by Stauffenberg for a military coup to seize power from the Nazis.

Now, however, the plotters were horrified to see that the 'Wolf' had survived. Hitler, battered, burned, and with 'a behind bruised as blue as a baboon's' as he jested - but still indisputably alive - staggered from the smoke of the shattered conference room, where four members of his entourage lay dead or dying.

Unsure of what to do next - they had not reckoned on Hitler surviving the bomb - the conspirators sent an ambiguous signal to Berlin: 'Something terrible has happened. The Führer is alive.' For the next three hours, as Stauffenberg - the head, heart and hand of the conspiracy - was in the air, the plotters in Berlin dithered, waiting for their charismatic leader to return. But by the time he got to the army HQ in the Bendlerstrasse - today renamed Stauffenbergstrasse in his honour - news was spreading rapidly that Hitler was indeed alive. Generals who had previously promised their support now drew back in fear of Hitler's revenge. SS men who had been briefly arrested on the plotters' orders in occupied Paris and Vienna were freed; and despite Stauffenberg's frantic efforts to shore up support by telephone, by nightfall the coup had collapsed. After a brief gun battle in which Stauffenberg took a bullet in the shoulder, he and his three closest colleagues were overpowered and immediately shot by firing squad in the Bendlerstrasse's courtyard. His last words were, 'Long live our sacred Germany!'

In 1944 Stauffenberg's wife, Nina, and her four children had been spending their summer holidays at the family's country house, Schloss Lautlingen, among the Swabian hills. The day after the bomb, young Berthold and his eight-year-old brother, Heimaren, heard a radio broadcast denouncing a 'criminal attack' on Hitler, but the adults shooed the curious boys away and evaded their questions.

Berthold remembers that long summer day with the clarity of a child's memory - the mounting tension before the storm burst: 'My mother's uncle Nikolaus, Count Uxkull - our "Uncle Nux" - was deputed to take us children on a long walk,' he recalls, sitting in his modest 1960s villa in the village of Oppenweiler, north-east of Stuttgart. 'He told us stories about his time as a big-game hunter in Africa to distract us. At that time, of course, none of us knew that he, too, was a member of the conspiracy. I often wonder today what thoughts were going through his head on that walk.' Uncle Nux was executed for his part in the plot a few weeks later.

On 22 July, Countess Nina told her two eldest sons that it was their father who had carried out the attack on Hitler. She added the news that she was pregnant with her fifth child. The boys were thunderstruck. 'Our world broke apart in an instant,' Berthold says, fixing me with an intense stare. He could no longer connect the two poles of his existence - caught between the father he adored, and the Führer and the Nazi system he had been taught to revere. 'I was a conformist and well on the way to becoming a little Nazi,' he admits. 'My dearest wish, which my mother fortunately prevented, was to march through our home town, Bamberg, carrying a flag at the head of the junior branch of the Hitler Youth.

'To my shocked questions about why my father had wanted to kill the Führer, she replied that he had believed that he had to do it for Germany's sake. This was so shattering for me that I don't think I was able to think clearly again for the rest of the war. We children loved our always-cheerful father above all things; he was our absolute authority - although he was often absent fighting the war - and now this! And then the blows fell thick and fast.'

The next day, Nina and Uncle Nux were arrested and taken to Berlin by the Gestapo. Even Nina's aged mother and aunt were seized, and the children were left in the care of their nanny, their grandmother's housekeeper, and two Gestapo officials. To deter acts such as Stauffenberg's, the Nazis had passed a decree known as 'Sippenhaft' ('kin detention') under which the families - including children - of those committing 'treason' against the regime were to be arrested and punished, too.

As they awaited their fate, cut off even from their friends in the village, the children feared for the future. 'On the radio and in the newspapers there were daily hate-filled reports about the conspiracy,' says Berthold. 'We - myself and my brothers aged eight and six, and my three-year-old sister - suddenly felt that we were alone. Outcasts. I will never forget that feeling.'

As an adult, Berthold's own military career included two stints as Germany's military attaché in London and as a result, he speaks fluent English with an Oxford drawl. Before he can continue the story, the phone rings. It is the military museum of his father's former regiment, calling him on some trivial matter. I sense that such calls come with unwelcome frequency, but Berthold is too polite and dutiful to refuse. As he courteously fields their enquiries, I take the opportunity to nose around his home.

Berthold's wife is away - preparing a party for the couple's golden wedding anniversary next month - and the only other inhabitant is Tanya, their black dachshund who scurries behind me, paws tap-tapping on the wooden floor. ?Various ancestors peer down at me from the walls, but at first I can see no sign of Berthold's father. Then, in a small corner, I come across a shrine to the great man. There is a large crucifix and a bronze bust of his handsome head and a couple of black-and-white photos. Taken on one of his last leaves, Stauffenberg has his arms around Berthold and his other children, smiling at the camera. An eye-patch covers his left eye, betraying the trauma of his recent battle in Tunisia but, apart from this, the soldier looks like a happy family man, at peace with the world. There is no hint of the cataclysm to come.

When I return, I find Berthold off the phone and preparing to eat lunch. 'Sit down,' he barks, directing me to a chair with a view of a hill crowned by a castle. 'Not one of ours,' Berthold says. 'But it was built by the Hohenstaufens, Germany's medieval rulers, from whom we derive our name.'

Lunch is served by Berthold's next-door neighbour. She respectfully calls our host by his noble title 'Graf'. We are given a homely Swabian pasta called 'spatzle'. As we eat, Berthold returns to the events of 1944.

On 16 August, 1944, nearly a month after the assassination attempt, Berthold remembers being taken, with his siblings, to a Catholic priest for his blessing. 'He told us that it was likely that bad times awaited us, but that we must never forget what our father had died for. Only later did I realise how brave his words were.'

The next day they were transported by train to a children's home at Bad Sachsa, near Nordhausen in the Harz mountains. The siblings were separated in chalets according to age and gender, and joined by the children of other conspirators. Although the director of the home, Frau Kohler, a Nazi party member, was 'strict and authoritarian', other staff members were 'outstandingly friendly', recalls Berthold. 'They never gave us the feeling that we were shunned by society.' Behind the scenes, however, darker forces were at work. Unknown to the children, plans were afoot to re-name them, since Hitler had decreed that the very name Stauffenberg should be wiped out. They were to be called 'Meister' and were apparently destined to be adopted by SS or loyal Nazi families. But before this, the children were to be sent to Buchenwald concentration camp; a fate they avoided thanks to the near-miraculous intervention of an Allied bombing raid.

'At Easter 1945,' says Berthold, 'my siblings and I, together with other conspirators' children, were put into an army truck and driven towards Nordhausen station, where we were to be put on the train for Buchenwald. But, just as we reached the town's suburbs, a fearful Allied air raid began which totally destroyed that quarter of the town, including the station. I heard later that a Nazi official who tried to restore order after the raid was lynched by the enraged townspeople. The war was nearing its end by then, and the party's authority was breaking down.'

The Nazis had no option but to drive the children back to Bad Sachsa, and a few days later, on 12 April, the American army liberated them. 'We had a grandstand view of the fighting as American planes - Mustangs and Lightnings - roared overhead,' Berthold remembers. 'Once the war came uncomfortably close when the strawberry patch outside our chalet was shot up.' But the children stayed put, as Hitler committed suicide in the Berlin bunker and Germany capitulated in May. ?They were cared for by two kindergarten nurses who had not fled with the rest of the staff. Suddenly, on 11 June, their great-aunt Alexandrine, who worked for the Red Cross, arrived with a bus to drive them home 'through a devastated Germany', as Berthold remembers, to Lautlingen, now in the French occupation zone.

The war may have ended, but for the Stauffenbergs - and millions of other Germans - in the starving, bomb-blasted, bereaved and ruined wasteland that Hitler had bequeathed them, the suffering went on. The children had to mourn their father, uncle, aunt and maternal grandmother (who had died of typhus in an SS camp) - and their mother was still missing.

Only in July 1945, exactly one year after Stauffenberg's bomb exploded, was the family unit reunited at Lautlingen. After her arrest, Nina von Stauffenberg had been interrogated by the Gestapo in Berlin's police headquarters, before being sent to Ravensbrück, the notorious Nazi concentration camp for women. From there she was taken to a maternity home at Frankfurt-on-the-Oder where she gave birth to her fifth child, Konstanze, in January 1945.

Fleeing westwards from the advancing Russians, mother and baby were both infected by a bug on a chaotic hospital train, and were brought, under Gestapo guard, to a Catholic hospital in Potsdam. After recovering, they were entrusted to a single policeman with orders to take them to Schönberg, where other 'Sippenhaft' prisoners were detained. 'But the cop saw such a "political" job as beneath him,' says Berthold. 'The war was ending and he just wanted to go home. So he simply abandoned my mother and her baby in a village - but not before getting her to write him a certificate stating that he had carried out his orders - very German! Here, they were found by the advancing Americans and thus she became the first "Sippenhaft" captive to be freed - although no one in conquered Germany really felt free'.

Berthold and his siblings, young as they were, had seen the worst of humanity in the space of one short year. And even in their rural retreat there was more to come. Given licence by their officers to plunder, the occupying French Moroccan troops ran amok in Lautlingen, looting and raping. The terrified villagers sought sanctuary with the Stauffenbergs. 'As the village "squires" we offered a measure of protection,' explains Berthold. Ironically, they were also forced to share their schloss with the families of the Gestapo officials who had been billeted on them.

Underlining the fact that inhumanity was not confined to the Nazis, Berthold saw the doomed Russian soldiers of the 'Vlasov Army' - turncoats who had fought alongside the Germans, and which his own father had helped to raise and equip - herded onto trains bound for Russia. What was their fate? 'At best Stalin's Gulag - at worst, a bullet in the neck,' he replies grimly.

Strange, given this history, that when the time came for him to choose a career of his own, young Berthold opted to follow his father into the German army. He reached this decision during the two terms he spent at Gordonstoun - the Scottish sister school of his German boarding school, Salem. Unlike the Prince of ?Wales, he enjoyed his time at the reputedly spartan school: 'I'm not particularly tough,' he says, 'but even so, I found Gordonstoun pretty soft.' He denies that his career choice had anything to do with his father - 'Even though I knew his shadow would always accompany me.'

'It was simply the right choice for me,' he insists. 'It was in my genes. And I was not disappointed - I never regretted it for one day.' But his father's shadow duly fell over him: 'I came to dread the question "Are you your father's son?"; after all, what is one supposed to say? But, especially in the early days, there were many in the army who had known my father, and there was always the unspoken question: why had they not chosen the path that he had? Why had they taken refuge behind the oath of loyalty they had all sworn to Hitler and used it as an alibi not to act?'

Berthold commanded a Panzer brigade and rose to head the army in the southern zone of Germany. Before he retired in 1994, he spent almost his entire career preparing for the war against the Russians that never came. Would he have fought them as his father had?

'Of course.'

Strolling among fruit trees in the orchard behind his villa - Berthold still owns 150 hectares of family farmland and forest in the Stauffenbergs' ancestral Swabian heartland - I ask why his mother, who died in 2006 aged 92, had never married again; she was only 31 when the war ended. 'First, because she worshipped my father and his word was law - what man could live up to him? Second, because there weren't many eligible men around in Germany after the war, and third, what man would want to take on bringing up five children?'

Stauffenberg sons, it seems, come in trios: his father had two brothers; Berthold himself has two brothers and he has three sons. 'None of them are soldiers. The oldest one is in IT, and married to an Iranian doctor; the second teaches, and the third lives in London where he is "something in the City".'

Berthold, as befitting the head of an extended clan which traces its roots back to the Middle Ages - at least 50 Stauffenbergs attend their annual reunions - is an aristocrat to his fingertips, with the antique but admirable attitudes of his caste. So is his wife, Mechthild, the Countess von Bentzel-Sturmfelder-Horneck von Oppenweiler (he calls her 'Maus' for short).

Nazi Germany, with its armies of thugs, crooks and time-servers was no place for such nobility. Before he was led out to be shot, Stauffenberg's penultimate last words, addressed to a secretary, were: 'They've all left me in the lurch.'

Despite his grumbles, the same imperative of duty dictates Berthold's tending of the family flame - such as his attendance at the annual ceremony on 20 July honouring his father's death in the Berlin courtyard where he and his companions were shot.

'Naturally, I've had to live with my father all my life; and I've always been compared to him - especially in the beginning by those who knew him - but this had less to do with 20 July, and more to do with his overpowering personality'.

As flamekeeper, Berthold has spoken out against the coming portrayal of his father by Tom Cruise in the film Valkyrie. Speaking to the German newspaper Der Spiegel last year, he said: 'It is unpleasant for me that an avowed Scientologist will be playing my father. I'm not saying that Cruise is a bad actor - I can't judge that. But I fear that only terrible kitsch will come out of the project. He should keep his hands off my father. He should climb a mountain or go surfing in the Caribbean. I don't care what he does, so long as he keeps out of it.'

Today, Berthold chooses his words more carefully, and will only say gnomically, 'We'll see what the film is like. I hope they don't play too fast and loose with the facts.'

But it seems unlikely that he will enjoy the film. For a noble Stauffenberg such as Berthold, other people's standards are never high enough.

ROUGH RIDE OF THE VALKYRIE

The most controversial film of Tom Cruise's career – before a single frame was shot

November 2006 Christopher McQuarrie, screenwriter of The Usual Suspects, presents the initial idea of a film based on the Valkyrie plot to director Bryan Singer.

March 2007 The duo pitch the project to United Artists partners Paula Wagner and Tom Cruise, who agree to finance the film, with Cruise to star as the plot's leader, Colonel Claus von Stauffenberg.

June 2007 The German government forbids filming on Bendler Block, Berlin, the site of Stauffenberg's execution. Officials state their disapproval of the 'dignity of the place' being violated. Berthold von Stauffenberg, interviewed in a German newspaper, declares that the film is 'bound to be rubbish'.

July 2007 Filming of Valkyrie begins in Germany. Release date set for 27 June 2008.

July 2007 Tom Cruise's affiliation with Scientology begins to receive attention. In Germany, Scientology is regarded as 'an organisation pursuing commercial interests' rather than a religion. Politicians, journalists and members of Stauffenberg's family express opposition to Cruise's role as Stauffenberg. The German Federal Film Fund grants 4.8 million euros to United Artists in order to assist with the production. Producers receive permission to film at Tempelhof International Airport's Columbia Haus, a former Nazi prison for political inmates.

August 2007 Ten extras are injured after falling off the back of a truck during filming in Berlin. One of the actors receives a serious back injury, while others suffer bruises, cuts and head injuries.

September 2007 The German government reverses its decision to forbid filming on Bendler Block. However, the production is denied a request to film at a Berlin police station.

October 2007 Footage is damaged beyond repair at a post-production firm in Munich. Production is forced to re-shoot several scenes.

November 2007 Tom Cruise is awarded a Bambi, Germany's most prestigious media award, for his role.

December 2007 MGM and United Artists announce that the release date will be moved forward from June to October 2008.

April 2008 The release date is postponed once more, to February 2009. Rumours spread that Cruise's German accent is comically suspect and that there are plans to re-record some of his lines.

May 2008 The Weinstein Company acquires the rights to a 2004 German film called Stauffenberg, based on the same material.

June 2008 More criticism inside Germany after claims that United Artists altered photos of Stauffenberg to make him look more like Tom Cruise. Alterations were reportedly made to the nose, chin and eyebrows. UA denies any tampering. A year after filming began in Germany, three scenes are re-shot in North Africa. Filming finally ends in late June.

August 2008 Early reviews of Valkyrie contain several revelations: there are no comedy German accents; one of Tom Cruise's hands is digitally erased; and it's 'pretty great'.