Sarah's Key by Tatiana de Rosnay is a work of fiction about a young girl, Sarah Starzynski, who was collected in France during the roundup of Jews in July 16, 1942, when the French police arrested thousands of Jews throughout Paris. The girl and her family were sent to an internment camp where people were held before being sent to Auschwitz where they were executed.

The story surrounds two time periods: 1942 and 2002. All of the main story and action takes place in Paris. The two stories alternate throughout the book. By alternating both points of view, de Rosnay allows the reader to learn factual information and also go on an emotional journey. As the book progresses, the stories tie together until they become one.

Julia Jaramond, an American journalist living in Paris, is assigned to cover the sixtieth anniversary of Vel' d'Hiv', a former stadium turned holding facility for Jews being sent to Auschwitz during the Nazi invasion of France. Julia has never heard of the Vélodrome d' Hiver, aka Vel' d'Hiv', which was located on rue Nélaton in the fifteenth arrondissement. Julia is told by her boss to get eyewitness statements and cover the real story of the site, which is an embarrassment to the French people. Unfortunately, Vel' d'Hiv' was torn down in 1959. Julia is told that it may be a tricky assignment as it is a touchy subject with the French. Most people know little, if anything, about the place or what happened on July 16, 1942.

Intertwined with Julia's research and near-obsession with Vel' d'Hiv' and the life of Sarah Starzynski is Sarah Starzynski's account of the events that took place in 1942. Although Sarah's portion of the story is told in third person, it is clear that the confusion, desperation and lack of understanding are seen from a child's point of view. Julia's account, on the other hand, is written in first person to give the reader the full emotional experience of Julia's research and attachment to a little girl who had most likely died sixty years earlier.

Sarah's story is told from the time of the arrest. Sarah's little brother, Michel, is locked in a cupboard to await Sarah's return. Sarah does not return and Michel dies. After being separated from her parents at Beaune-la-Rolande, Sarah manages to escape. She and her fellow escapee, Rachel, end up staying at a farmhouse in Orléans. Rachel is captured but Sarah is adopted by an elderly couple who raise Sarah as a grandchild.

Julia works tirelessly to learn about the people that had been corralled at Vel' d'Hiv. Julia learns that her new apartment was once lived in by the Starzynski family and that there is a terrible secret tied to the roundup known as "Operation Spring Breeze".

Julia learns about Sarah and about the death of Michel. Julia tries to track down Sarah, only to learn that she had died in 1972. Later, Julia would learn that Sarah could no longer bear the burden of her past and committed suicide. Julia tells Sarah's son everything about his mother. The son, William, is shocked because he knew none of it.

During Julia's quest to find Sarah, she realizes that her marriage is falling apart. A pregnancy seals the deal and Bertrand leaves his wife of sixteen years.

Julia and her daughters end up in New York City where Julia desperately tries to start a new life. The book ends with William contacting Julia, and the pair meets once again to talk about Sarah.