Elle s'Appelait Sarah," like so many films these days, uses the device of multiple narratives. First you've got a story set in the present, where journalist Julia (Kristin Scott Thomas) is investigating a bit of repressed French history: the 1942 roundup of Parisian Jews, where thousands of people were imprisoned in the Vel d'Hiv stadium for a week before being shipped off to Auschwitz. The pogrom was notable for having been carried out by French police, not the Nazis, and for taking place a stone's throw from that symbol of Parisian allure, the Eiffel Tower.
Then the film (released in English-speaking territories as "Sarah's Key") skips back to the 1940s for a re-creation of these terrible events, as seen through the eyes of a young Jewish girl named Sarah (Mélusine Mayance), whose family are evicted from their home and sent to the stadium as neighbors look on silently.
The meeting point between these two stories comes when Julia begins to suspect that the apartment owned by her in-laws — which her husband has restored for them to live in — may have originally belonged to Sarah's family.