Sarah's Key is the story of two women, the young French-Jewish girl Sarah Starzynski and American immigrant to Paris Julia Jarmond. Sarah's story tells of her capture by Nazi officers and French collaborators and of her journey back to her home where she had innocently locked her younger brother in a closet, not knowing she was to be abducted. The boy dies, and decades later the journalist Julia investigates Sarah's story as a means of finding her own identity.
The first plot follows the Starzynski family. On 16 July 1942, French police raided the Starzynski apartment in Paris, arresting ten-year-old Sarah and her parents. First, the family is sent to the Vélodrome d'Hiver, an enclosed stadium that housed a bicycling track for racing. The French held more than 7,000 Jews, mostly women and children in the stadium, which was made to hold far fewer people, and they were eventually sent to Drancy, a refugee camp, to separate the men from the women and children and then later, they separated the mothers from the children.
Later, we find out, they were all sent to Auschwitz. Sarah, now on her own, is still tormented by thoughts of her brother locked in the secret closet at home, and tries to escape with Rachel, another Jewish girl at the camp. They are caught, but the officer that catches them is a man that Sarah knows from her old life. She begs him to let her go to rescue her brother. He reluctantly helps them escape and even gives the two girls money.
The girls find refuge with an elderly couple who takes them into their house, but Rachel falls ill with what is probably diphtheria or typhus fever. The couple, Jules and Geneviève Dufaure, are compelled to call a Nazi doctor, who reports to the Nazi officials that the elderly couple is hiding Jewish children. The Nazis arrive and search for more children, but without finding Sarah they take Rachel away and she presumably dies.
Jules and Geneviève dress Sarah in boy's clothes (her hair had been cut short at Drancy) and help her get to her former home in Paris. She had been keeping the key to the closet in her pocket the whole time. She knocks at the door, screaming her brother's name. A young boy or Eduoard as a child (Bertrand's father)opens the door and Sarah rushes into the house, nearly kicking the door to her former room and quickly opens the closet. She collapses and starts to cry hysterically. The boy's father, or Edouard's father comes into the room and removes Michel from the closet and laid him on Edouard's bed, who haddied by then (shriveled up), hidden in the closet waiting for Sarah.
The second plot follows Julia Jarmond. She was born in Boston, Massachusetts but moved to Paris in her early 20s and married a Frenchman named Bertrand Tezac and had a daughter, Zoë. Sometime prior to the action of the book, Bertrand had cheated on Julia with a woman named Amélie after Julia had suffered a miscarriage. The apartment they are thinking of moving into happens to have belonged to the Starzynski family prior to being inhabited by Bertrand's grandparents and father, who was then about Sarah's age, but neither Julia nor Bertrand are aware of this.
Julia's boss, Joshua, asks her to write an article about the Vel' d'Hiv Roundup, as the 60th anniversary was nearing. Julia tries to find out more about the roundup and through her research and questioning of Bertrand's grandmother discovers whom the apartment once belonged to. When Julia finds out and tells Bertrand, he does not seem to care.
Julia becomes obsessed with the story of young Sarah Starzynski, who was recorded to have been taken to Beaune-la-Rolande, but was not recorded to have returned nor taken to a gas chamber. She does more research and finds out she ran away, but she loses trace of her after that. She does a little more research and becomes in touch with descendants of Jules and Geneviève and they tell her that Sarah had continued to live with the family into her teens, but she grew up an introverted, sad young woman. Julia finds out she is pregnant, but Bertrand says she must either choose between an abortion or a divorce. She chooses to have the baby.
She discovers that Sarah married and had a son. Sarah had died when her son William was a young boy and Julia arranges to meet him in his home of Italy. He is very reluctant to learn of his mother's story and asks that Julia not contact him again. Weeks later, the same night Bertrand's grandmother has a stroke, William shows up in France, finally ready to hear Sarah's story. After Julia delivers her daughter she and Bertrand part ways; Bertrand moves in with Amélie and Julia does not feel comfortable about moving into the apartment they were to have moved into, with its sad history of Sarah, Michel and their parents. She moves to New York City with Zoë and the baby.
William, having uprooted his own life too in the wake of everything he has learned about his mother, also moves to New York City and eventually tracks down Julia again. He shares with her some of Sarah's writings that he found in his father's drawer. It is revealed that Sarah did not merely die from a car accident as previously believed, but had killed herself intentionally, unable to bear with the pain of her secret past any longer. When William asks Julia what the baby's name is, she replies 'Sarah'.