The Nightingale, by Kristin Hannah, is about two sisters struggling to survive during World War II in France. One sister is rebellious and intent on fighting for France while the other simply wants to survive the war with her family intact. Both sisters learn who they are and what they are capable of as the war wages on.

The Nightingale begins at the Oregon Coast in 1995 with an elderly woman who is dying of cancer. Her son is helping her to move to a retirement home. Her identity is not revealed. As her son is helping her, he sees a photo of a woman named Juliette Gervaise and asks his mother who she is. The elderly woman begins to remember the story of Vianne and Isabelle.

The main plot begins just as France is being drawn in the war. Vianne, the older sister, lives in the town of Carriveau where there is an airstrip. She is married and has one daughter, Sophie. Vianne enjoys her quiet life and relies on her husband, Antoine, to take care of everything. Isabelle, the younger sister, has been living in one boarding school after another because her father is incapable of raising her and Vianne was unable to do so after suffering a miscarriage. Isabelle is impetuous and has run away from or been expelled from several schools. Isabelle has just been expelled from another school and goes to Paris to convince her father to allow her to live with him.

When France is drawn into the war, Antoine is sent to fight as a solider as is Vianne's best friend Rachel's husband, Marc. The two women are left alone to raise their daughters and continue their teaching jobs at a school at which they both work. Vianne is certain she'll be unable to carry on alone.

Julien, Isabelle and Vianne's father, sends Isabelle away with the Humberts to escape Paris as the Nazis invade. She is to return to Carriveau to live with Vianne. When the car runs out of fuel, Isabelle begins to walk and meets a young man named Gaetan, with whom she falls in love. Gaetan invites her to join the French resistance, but leaves her behind in Carriveau. Isabelle lives with Vianne, but remains outspoken, which Vianne fears will draw the attention of the Germans and cause them harm.

A Nazi named Beck billets with Vianne. He treats Vianne and Sophie kindly even when Isabelle speaks up. Isabelle joins the resistance movement and begins distributing anti-German tracts. This work leads her to move back to Paris where she becomes a courier for the resistance movement.

Vianne continues to teach alongside her friend Rachel. When Beck asks Vianne for a list of names of Jewish people and other undesirables who work at the school, she is forced to give him Rachel's name. Rachel is fired from her job. As the war continues, rations become scarce and the winters are very difficult. Beck informs Vianne that Antoine has been placed in a prison camp.

One night Isabelle finds a downed Allied airman in Paris and brings him to the resistance group. She devises a plan to transport airmen across the Pyrenees mountains into Spain where they can safely rejoin their military forces. From that point on she becomes the Nightingale and is in grave danger of being captured by the Nazis. She is given the false identity of Juliette Gervaise to protect her family and friends.

Beck tells Vianne that she should hide Rachel because the Jews are being rounded up and sent to work camps. Vianne hides Rachel in her barn cellar, but not long enough. As they try to escape, Rachel's daughter Sarah is shot and killed. When Rachel is taken she begs Vianne to take her son, Ari. Vianne takes Ari and passes him off as a child named Daniel that she adopted from her husband's cousin.

Isabelle returns to Carriveau. When the airstrip is attacked she rescues another pilot and hides him in Vianne's farm. Beck discovers Isabelle and Vianne hits him in the head with a shovel while Isabelle shoots him. Isabelle has also been shot. Beck dies. Gaetan arrives with other resistance members and they take Beck's body and the body of the pilot away. Gaetan also takes Isabelle to a safehouse to heal. Isabelle and Gaetan finally admit their feelings and begin a love affair.

Another Nazi named Von Richter billets with Vianne. He is very cruel and rapes Vianne, getting her pregnant. Vianne accepts the son of another Jewish friend who is being sent away. She knows she can no longer stand by and watch the evil around her so she takes the boy to the orphanage at the convent. Mother Superior Marie-Therese tells Vianne she will help hide more children, so Vianne begins to rescue Jewish children whose parents are taken away.

Isabelle is captured by the Nazis but refuses those who question her about the identity of the Nightingale. They don't think she is the Nightingale because she is a woman. In order to save Isabelle, her father turns himself in claiming to be the Nightingale. He is shot and Isabelle is sent to a work camp.

When the war ends, Antoine returns home to the pregnant Vianne who tells him that she must have conceived on the first night he came home. Isabelle returns to Carriveau, but is extremely ill. Gaetan comes to Carriveau to find Isabelle and she dies in his arms. Jewish officials come to take Ari, now called Daniel, away from Vianne to return him to Jewish relatives living in America.

The novel ends with the elderly woman being revealed as Vianne. She is at a reunion of French resistance members where she tells Isabelle's story to the audience. Her son, Julien, is with her and learns about his mother's past for the first time. Ari comes to Vianne and tells her he never forgot her or Sophie and has been looking for them. Gaetan also approaches Vianne and introduces his daughter named Isabelle. Vianne tells her son that she will tell him everything, but she keeps the secret of the events surrounding his conception.