We are then taken to the present day where American journalist Julia (Kristin Scott Thomas) is writing an article on the round-up, before realising she has an intimate personal connection to Sarah and her family. It leads her to obsess over the fate of Sarah as her own family life crumbles around her.
Director Gilles Paquet-Brenner has personal reasons for making the film. He lost members of his own family to Vel d'Hiv, and grew up dismayed at his native country's unwillingness to face up to such a visceral scar on its national conscience. "The story resonated with my own family history," he says.
"I'm of Jewish origin and the men in my family were victims [of the round-up]. My grandfather, a German Jewish musician who had settled in France, was denounced by some French people and died shortly after being sent to the camps.
"My mother told me that story for the first time while I was in pre-production for the film. Certain things resurfaced. Obviously, I wasn't around when my grandfather was deported, but I saw how it had affected my grandmother and my mother and her sisters. The book brought that back to me - the living who have to learn to live with the dead.
"I pay tribute to him in the film through the character of the violinist who has a ring containing poison so he alone can decide when he dies."
Given this highly personal connection with his subject, the urge must have existed to render the story in moral extremes - to show Jews as victims and the French people as reticent or willing Nazi scions. But Paquet-Brenner was led by the novel to create a film of shades, of people both selfish and giving, good and bad.
"I was dazzled by de Rosnay's plot, which explores the grey areas that few films deal with – like the attitude of regular people during the roundup – that were not framed in terms of collaborators and resistance fighters. The silent majority averted their eyes and tried to save their skins. Some didn't do anything wrong and don't feel at all guilty. Others become heroes almost against their will.
"We're not in a good-versus-evil scenario - we have the facts and the consequences. We're miles away from the usual shortcuts and simplifications."
Recreating and immortalising history, he says, "terrified him," but the care and attention with which Paquet-Brenner has recreated Paris of 1942, and in particular the infamous Winter Velodrome where the Jews were sent to wait for deportation to the camps, is arresting.
Keeping CGI and special effects to an absolute minimum, we see the velodrome purely from the perspective of the delirious Sarah. "I prepared for the scene by meeting survivors whose recollection were of the constant stifling heat, noise, smell and teeming crowds.
"I wanted audiences to get a sense of the vastness of the velodrome without being too demonstrative. I was wary of digital effects that let you do whatever you want, sometimes at the expense of realism. Every shot in the velodrome is from Sarah's point of view."
It was, he says, an ordeal to shoot.
"At first, I couldn't capture the essence of what I was witnessing - the barbarity of it - and I started to get worried and frustrated. Gradually, I moved the camera closer and I asked the cameraman to go in among the crowd, even if it meant the extras bumping into him or jostling him. He took some knocks but in five takes we had captured the chaos as you see it on screen."
Fittingly, Sarah's Key was the first feature film to shoot at the Holocaust Memorial in Paris. To do so, Paquet-Brenner acknowledges, constituted a risk.
He says: "We could so easily get drawn into something political," he says. "The man Julia meets in the museum sums up his mission as, 'getting away from figures and statistics to give a face and reality to each of these lives'. Those words define my underlying aims with the film. Until now, films about the Holocaust have stuck close to history with a capital H. That's understandable and indispensable, but I didn't feel comfortable with that.
"It's been done so many times and, to my eyes, Schindler's List just can't be bettered. So I wondered how I could make my modest contribution, and what I hit upon was trying to make people feel the tragedy by making it palpable, so that audiences would feel in contact with the events. Kristin's character Julia is American and non-Jewish, so Sarah's story and the Holocaust is not her story, but indirectly it touches her. It could happen to anybody."
