Soon after noon on July 20 1944, a bomb exploded in the Wolf's Lair, Adolf Hitler's military headquarters in East Prussia. The intended target was the Fuhrer.
If the plan to kill Hitler and launch a coup had been successful, Carl Goerdeler would have taken over as leader of the new Germany.
Colonel Claus Von Stauffenberg, a dashing war hero who had placed the bomb in an attaché case in Hitler's briefing room, was to be Minister for War.
And my grandfather, Fritz-Dietlof Von Der Schulenburg, was set to become Minister of the Interior.
As the bomb ripped through the barracks shed, killing four people and wounding others, Fritz was ready and waiting in Berlin for the seizure of power. It was my grandmother's birthday on that day, but he could not celebrate it with her.
As the historian Ian Kershaw put it: "What could go wrong for the plotters on that day did go wrong.'' Stauffenberg was forced to hurry when he was preparing his bomb in a side room, because he was called away to a phone call.
There was no time to set a second explosive, which would have doubled the impact. Also, a heavy oak table leg helped to shield Hitler from the blast.
Hitler staggered out of the hut with nothing worse than splinters, minor cuts, and burst eardrums.
In the grand scheme of things, the failure of the coup meant that the war continued for another 10 months. On a personal level, Valkyrie shaped the destiny of all four of my grandparents, and brought about the eventual meeting of my parents in Ireland.
Both Fritz and my other grandfather, Peter Bielenberg, were arrested.
Fritz, a Prussian count, had been a relentless plotter against Hitler, involved in several assassination plots before Valkyrie.
As vice-president of the Berlin police in 1938, he planned to have the gates opened to the chancellery in Berlin so that a commando unit of resistance fighters could seize Hitler. This plot was shelved along with many others.
In 1940, Fritz hoped to have Hitler shot during a victory parade through Paris in late July. Sharp-shooters were to be placed in the crowd. But the elusive Nazi leader never turned up.
Early on in his career, Fritz had been an enthusiastic Nazi. He hoped the National Socialists would restore the pride of a country humiliated in the First World War. But he was soon disillusioned by the corruption of the new regime.
It was only in 1943, after Stauffenberg was badly wounded on the North African front -- losing his right hand, two fingers on his left, as well as his left eye -- that he emerged as the driving force of the Resistance. Stauffenberg sent an electric charge through the secret opposition.
Fritz and Stauffenberg spent their last Easter together at my great aunt's country home at Trebbow in north Germany. Fritz's family, including my mother Charlotte, then only four, had been evacuated there to escape the air raids on Berlin.
During that Easter, the family told ghost stories. The men discussed Shakespeare, and their plans for the future. Stauffenberg's driver, a magician in civilian life, performed magic tricks for the children.