As a young child, Niklas Frank had only a vague understanding of his father's standing.
He remembers idyllic summers in the early 1940s, when his family travelled back and forth between their home in Germany and their castle in southern Poland.
Black-and-white videos from those years show a confident, carefree family in bucolic surroundings. He remembers driving his toy car into the legs of the various stern-faced, military-uniformed adults who visited his parents from time to time; and barking commands at the castle's servants, who were too afraid of Frank's father to admonish him.
"I knew Poland as our private property," Niklas remembers. "This, I thought, was quite normal."
It was only once he was a little older that Niklas truly understood his father's dark role in European history - as Hans Frank, "the Butcher of Poland".
Serving Hitler between 1939 and 1945 as governor-general of occupied Poland, he was later held responsible for the murders of three million Polish Jews, many of whom died in agony in the gas chambers at Auschwitz. He was convicted and hanged by the Allies in 1946.
Niklas's family history, which he had first learnt of that same year after seeing photos of "mountains of corpses" at the death camps in Poland in US newspapers, remained a secret until 1987. That was when he renounced his father in a best-selling book, Der Vater: Eine Abrechnung (The Father: A Settling of Accounts).
The book shocked post-war Germany, the 81-year-old says, a society still grappling with the demons of its Nazi past. Described as a scathing "hate letter" to his parents, it will be published for the first time in the UK this year (a heavily abridged version was published in English in the 1990s, without the acid-tongued emotion that made it so provocative in Germany).
"I think the strongest taboo around the world is: 'Don't attack your parents,' " Niklas says over Zoom from his farmhouse near Hamburg.
In spectacles and a red jumper, the retired journalist is both cheerful and strikingly direct, refusing to shy away from even the grisliest aspects of his father's legacy. He shows nothing but scorn for the man who once told Nazi accomplices: "We must annihilate the Jews wherever we find them."
Niklas says: "He was a coward, a bloody careerist. As a youngster he experienced the glorious Reich, then suddenly after the First World War everything was gone: big inflation, democracy.
"My father wrote in his diary: 'Germany needs a powerful man who rebuilds our honour.' Then he met Hitler and he really fell in love, I would say. He always wanted to please Hitler."
The youngest of five children, Niklas was 6 months old when his father, who had worked in the early 1930s as Hitler's personal lawyer, became Poland's governor-general.
He remembers Hans as a jovial but unaffectionate father who used to switch off the living room lights and play opera while he told his children scary stories of robbers hiding in forests. One morning, at their Polish castle, Frank walked into the bathroom to find his father shaving; he turned around and splodged foam on Frank's nose - the only warm encounter he recalls.
His mother, Brigitte, who dubbed herself the "Queen of Poland", would take shopping trips into Krakow's Nazi-built ghetto - home to 50,000 Jews until 1943, when its inhabitants were exported to death camps - and use her position to accrue fur coats, handed over for free by the residents who feared her.
Once, aged around 4, Niklas was taken along; he remembers sitting in a taxi while his mother shopped, as down-trodden Jewish families walked past. Frank stuck his tongue out to one older boy.
"He didn't react, he just went away. I was laughing in full triumph that I'd beaten him. But Hilde [the nurse] drew me back. I didn't realise then it was a ghetto. That scene is burned into my head."
His parents despised each other, so much so that at one point in the war Hans wrote to the Fuhrer himself requesting permission for a divorce. Hitler warned him it could jeopardise his standing; Hans obliged and remained miserably married.
Niklas was largely sheltered from the war, except for one occasion in Poland when he saw a tank burning in the street (it had probably been attacked by Polish resistance fighters, he thinks).
As the Allies closed in, in spring 1945 Hans invited two of Niklas's elder siblings into his office in Germany: "My father said to them: 'I think I'm the last Reich minister who can ever offer coffee and cakes in freedom.' "
Soon afterwards, an American jeep pulled up to the house. Hans was arrested, and later tried twice to take his own life.
Niklas barely heard from his father during his year-long trial at Nuremberg, but they did visit him in prison in September 1946, weeks before he was sentenced to death. Sitting on his mother's lap, aged seven, he remembers speaking to his father through a glass panel, watched by an Allied soldier. A lawyer had already told the family that the overwhelming evidence against Hans gave him little chance of survival.
"I sat there knowing it was my last visit, and he lied to me. He told me: 'Nikki, we will soon celebrate a happy Christmas.' I was thinking: 'Why is he lying, he knows he will be hanged?' I was so disappointed."
The family lost all of their money in the war; in the years after Hans's execution, a teenage Niklas was sent with his brothers to boarding school near Munich (funded by his father's surviving Nazi friends, he thinks). On weekends, he hitchhiked home to his mother; sometimes, he bragged in the car about his family name.
"It really makes me red-faced. I used to say, 'Do you know who I am, by chance?' In the early 50s the cars were always driven by old Nazis and they'd say: 'Oh, poor boy, your father was innocent, have you got enough money, have you got something to eat?' I enjoyed it. But one day the driver stopped immediately and threw me out ... I never tried it again."
He made an exception for his daughter Franziska, now 54. He was researching Hans's life and shared many of his findings. "She grew up with it. Very often I'd come home and say: 'Franziska, I have a new document, look at what your grandfather did'." Franziska went on to study history at Cambridge and now lives in Berlin with her three children.
It was Germany's attempt to distance itself from its historical demons that ultimately prompted him to come clean about his father in 1987, he says. He regards it as propaganda.
"Never in Germany came an urge from the people to build memorials for Holocaust victims. It was always from politicians. The silent majority was never happy. I'm the only descendant of a big-shot Nazi who opened up" - something that led to death threats from neo-Nazis when his book was first published.
He thinks there is historically a "direct line" from Nazism to Alternative fur Deutschland, the far-Right, anti-Muslim party that now controls 12 per cent of seats in Germany's federal parliament.
His honesty caused a rupture with his siblings. One sister, Sigrid, emigrated to South Africa, while his sister Brigitte tragically took her own life in 1981. Niklas is the only one who remains.
Aware of the dwindling number of Holocaust survivors, Frank is also keen to help his three grandchildren understand their great-grandfather's dark role in history, so they prevent it from happening again - a lesson for their whole generation, as much as it is for them.
"Franziska told them very slowly, but we never hide anything. If my book is lying around and [they're] asking, we always answer. I never try to hide anything. Because they're not guilty, their great grandfather was."
He wants them to know that heritage is not destiny. Under the right circumstances, he says, the apple can fall far from the tree.