When the director Sam Mendes was a young boy, he and his father often traveled to the West Indies to visit his grandfather Alfred Mendes, a novelist. Sam, who had been brought up in North London, found his grandfather to be quite exotic: The small and wiry World War I veteran would sing opera in a booming Trinidadian accent, traipse around his creaky Colonial house in shorts and flip-flops and vigorously greet each morning with a pre-dawn plunge into the sea.

Alfred Mendes also had a tendency to obsessively wash his hands, always for several minutes at a time, to the point where Sam and his cousins noticed that above all his other quirks. “We would laugh at him,” the director recalled, “until I asked my dad, ‘Why does Granddad Alfie wash his hands so much?’ And he said, ‘Oh, he remembers the mud of the trenches during the war, and the fact that he could never get clean.’”

That’s when the boys stopped laughing at their grandfather. It’s also when they began asking what happened when, at age 19, Alfred Mendes enlisted and fought on behalf of Britain in what would become one of the world’s deadliest conflicts.

“We expected, I suppose, conventional stories of heroism and bravery,” Mendes said. “We certainly didn’t expect what he told us, which was unbelievably shocking and quite graphic tales of utter futility and chaos.”