If this movie wasn't a hit novel, I'll eat my hat – it has all the elements bestseller lists love. First, a woman on a mission to right a historic wrong. Second, concurrent timelines with two female characters. Third, Nazi and the Second World War. And fourth, a tiny talismanic plot device the whole thing hangs on.

In this case it's the object of the title, a play on words as reporter Julia (Thomas) tries to track the life of a little French girl during the war – she's trying to find the key to who Sarah was as well as the story literally hinging on the key Sarah is carrying with her.

Looking into a pivotal event in wartime Paris, Julia realises her family home might have a connection to an anti-Semitic atrocity and as she digs deeper, Sarah's story starts to appear.

It's 1942 and the French, after virtually lying down to invite the Nazis in on a red carpet, have started to betray the Jews among their own people. When French police rounded up thousands, they were kept in a sports stadium for days before being separated and sent to death camps.

Sarah is holding a key because when the secret police pound on her door she's at home playing with her little brother. Their mother answers the knock by the fearsome detectives and Sarah hides her little brother in a secret compartment in the bedroom, locking the door and promising him she'll be back for him.

As she and her parents are shipped off and left to rot in the stadium before being split up and shipped to camps, Sarah and another girl finally escape and begin the long journey back to Paris to let her brother out.

In the present, Julia is trying to piece what became of Sarah together after the latter grew up (never seeing her parents again) and eventually moving to America and marrying.

After both stories play out, the present-day narrative ends up not quite knowing where to do, the script seeming to think that after all you've learned the only thing you need is a big warm hug of acceptance and closure.

I've also read that putting both stories side by side renders Julia's plight a mess of first world problems, which to be honest is a fair complaint. You might expect that what Julia learns about what went on puts her own problems into perspective, but they don't seem to.

Without the entire present day story it would have been an informative and heartfelt saga of something we should never forget. With it included, it gives the story narrative drive by using a detective story aspect, but doesn't really add to the drama. The whole thing also feels slightly leaden in tone and mood – not inappropriately considering the subject matter, but a sad drama doesn't have to be so stilted.