Flanagan grew up in Tasmania, the descendant of Irish convicts sent to Van Diemen's Land for stealing cornmeal during the famine. "They intermarried for a hundred years," he says of his ancestors. "There was at once an enormous memory of certain things, and a great forgetting of other things. And there was a great and determined forgetting of the experience of being convicts."
Flanagan's was the first generation to receive a further education, yet even so, he left school at 16 hoping to learn a trade and write in his spare time. "My mother hoped I'd be a plumber."
He didn't get a trade, as it turned out, but found a job as a bush labourer - "a surveyor's chainman, a job that doesn't even exist now" - and after that he drifted back, as he puts it, to university. The drifting continued via a Rhodes scholarship to study history at Oxford, but the only thing he ever took seriously was writing, he says.
"Perhaps the virtue of coming from a place like Tasmania," Flanagan suggests, "is that you had the great gift of knowing that you were not the centre of things, yet life was no less where you were." A contented outsider, he read, at the age of 11, a book that mirrored his position and showed him what a novel could be: Camus' The Outsider. Even then, when he "understood almost nothing of adult life", he sensed the way in which a violent act could feel like an innocent one in the heat of the moment, and how the effects of the sun could turn a character on its head.
Flanagan's father was born in 1916. By the time he returned from the war and met the woman who would become Flanagan's mother, he was 31. They had six children, and Flanagan knew him as an old man - a sick man, indeed, and one who had "strange anxieties".
"My father, unusually for a PoW, talked about his experiences, but he talked about them in a very limited way," Flanagan remembers now. The book is dedicated to "Prisoner san byaku san ju go" - Prisoner 335, and these Japanese words, he says, were "words I knew for as long as I can remember". The book is so close to his own experience that, despite its setting, Flanagan doesn't see it as a historical novel; he sees it as a contemporary one.
"I grew up not simply with the knowledge of what had happened to my father, but with something vastly more incommunicable: some idea of life - humanity - that had come to him, and had passed to us, his children," he explains. "And this idea was so large and strange, and so inexplicable, that it began to almost choke me. I didn't wish to write this novel - because it was so pregnant with failure and catastrophe, and because there were other books I wanted to write - but I knew that if I didn't write it, at a certain point I wouldn't be able to write anything else."
The five early versions he wrote were wildly different from each other, and he burned them all. "And then my father was growing old and frail," he recalls, "and though the things aren't connected at all, except illogically in my mind, it mattered to me to try to finish it before he died. And so I returned to it, but conceiving of it now as a love story."
The book takes as its own title that of a 17th century Japanese epic poem, a famous work by Basho. This was as much a mental exercise as a nod to that tradition. "I wanted to use what was most beautiful and extraordinary in their culture in writing a book about what was most terrible," he explains, "because I thought that might liberate me from judgment. And it did help me."
When he had almost finished, Flanagan went to Japan. It was a trip he had been putting off, and one his father dreaded on his behalf, but he knew some of the Japanese generals and other camp staff were still alive, and he wanted to speak to them.
He describes three extraordinary encounters. A former medical orderly told him that "when he first arrived at my father's camp, they were just skeletons crawling around in the mud, and that it looked to him like a Buddhist vision of hell". Flanagan pauses as he says this, and stumbles a little over his words. "I can't tell you how terrible I felt."
The second encounter was with a man who was his father's nemesis - nicknamed "The Lizard". He had been sentenced to death for war crimes, but had since had his sentence commuted to life and had changed his name. Flanagan didn't realise who the man was until five minutes before they met.
For reasons that Flanagan now describes as too complex to go into, he asked The Lizard to slap him across the face repeatedly - a punishment that had been meted out routinely to his father. The Lizard began, and on the third slap an earthquake shook Tokyo. Flanagan didn't know what was happening, but when he saw the fear in the old man's eyes, he says: "I knew that wherever evil was, it wasn't with us."
The third encounter was strangely gentle. He travelled further south to meet a former guard, and by this time the media were following him. They asked Flanagan to pose for a photograph with Mr Seiko, "a tiny cockatiel of a man", and the small sometime torturer put his arm around Flanagan and wouldn't let go. "He curled into me, the way little children do when they want forgiveness," Flanagan reflects, "and I realised there is strangeness in this world beyond any understanding."
Flanagan's father didn't live to read the novel. But then, Flanagan says, he probably wouldn't have read it anyway. "He simply trusted me, that I wouldn't get it wrong. And now that he's passed away I can say that was the greatest gift."