14/08/2011 07:39 BST | Updated 14/10/2011 06:12 BST

Interview: Gilles Paquet-Brenner, Director Of Sarah's Key, Starring Kristin Scott Thomas, About Vel d'Hiv (VIDEO)

Knocks at the door in the dead of night, families rounded up and forced to make their way on crowded trains bound for Europe's ominous heartlands, loved ones separated and secrets of guilt and betrayal passed down through generations - this may be the standard stuff of Nazi wartime history, but there is an even more sinister twist - these were actions inflicted on the French, by the French, and will come as a shocking surprise today, even to millions of the country's own citizens.

This shameful chapter is now the subject of an ambitious and moving film, Sarah's Key, by French director Gilles Paquet-Brenner, whose own Jewish family was fragmented by the Holocaust, but who admits his own knowledge of events was scarce.

"Before I read the book, (Elle s'appelait Sarah by Tatiana de Rosnay) I knew the expression Vel' d'Hiv Roundup, but like millions of French, I didn't know what was behind this word. We know without really knowing."

The Vel D'Hiv Roundup refers to a horrific but comparatively untold chapter in wartime history, when French collaborators rounded up Jewish families, sent them to Paris's Velodrome d'Hiver stadium, and onwards from there to the camps. Then French president Jacques Chirac publicly apologised for these events in 1995, and it was only then that general awareness developed.

The film, critically lauded since its first showing in October 2010 and endorsed by the influential Holocaust Museum in Washington, sits in the present - with Kristin Scott Thomas in a French-speaking role as a journalist Julia based in Paris with her French family - but delves into the past as she uncovers the story of young girl Sarah, who struggles to escape the Vel D'Hiv, fuelled by her mission to save her younger brother Michel.

"The two timeframes bring the history into context for our audience," explains the director. "It can be really boring to delve into history, so it's good to show how the past affects our present, and can shape what we are today.

"But I was anxious because, on one side in 1942, you have this incredibly strong story, so I was worried that the present would be less interesting.

"But you actually need the scenes in the present to catch your breath. And Julia brings it to a modern audience."

Scott Thomas has been critically lauded for her work in this film, for which Paquet-Brenner describes her as a natural fit:

"Kristin has been living in Paris for about 30 years now, so she is very credible to French audiences as a wife living with a French family - that's basically her life.

"I didn't know her, but I sent her the script - she was working on Broadway, and her daughter Hannah threw the screenplay on her desk, and told her to read it because the book was so good.

"I met her in New York on Obama's Election Day, which made it a very special day, and she said yes the day we met, which is pretty rare."

Paquet-Brenner is all too aware of the responsibility of bringing it to a generation of French unaware of its own traumatic past:

"There haven't been films about the Vel d'Hiv before. After the war, the big urgency was to realign French society so we could live together again. Looking back came later in the 1960/70s, but first the collaboration had to be dealt with. It's the big French shame - for 30 or 40 years after the war, the question that can't be asked is 'What did you do?'

"It certainly would have been very different 20 or 30 years ago, because some of the people involved would have still been in senior roles."

Now, though, Paquet-Brennier is convinced his film can only help the French move on:

"Guilt is a very dangerous tool, I don't want to use that. Instead, it's about knowing the facts, possessing them and realising everyone's capacity both for good and ill. It's human nature, and that's what is enduringly fascinating."

Sarah's Key is now showing in selected cinemas nationwide: