The novel opens with Elena Greco learning that her longtime friend Lila Cerrullo (also known as Raffaella and Lina) has vanished without a trace. This strange turn of events prompts Elena to begin writing the history of her relationship with Lila, beginning with their childhood in Naples, Italy, in the 1950s.
Elena and Lila meet in grade school. They both live near each other in a rough and tumble, working-class neighborhood where violent feuds are common, and the lives of many families intersect through shared stories and secrets. Lila is precociously intelligent and courageous, attracting the attention of the more mild-mannered Elena. They quickly become close friends although Lila's more assertive temperament means that she can often make Elena do her bidding and push her beyond her comfort zone. Lila's inquisitive and observant nature leads to Elena being profoundly shaped by some of the events the girls witness and discuss, such as the affair between Donato Sarratore and the mentally unstable widow Melina Cappuccio, or the murder of Don Achille Carracci.
Elena's and Lila's intellectual talents are recognized by their teachers, and at the end of the fifth grade, it is recommended that both girls take the exam that would qualify them to continue their studies in middle school. Elena's parents are uncertain, but reluctantly agree to pay for the private lessons to help her prepare for the exam. Lila's parents, however, refuse, despite the objections of her brother Rino, and Lila is sent instead to complete some courses targeted to a more technical education.
The beginning of Elena's studies in middle school marks a shift in the friendship between the two girls, as Lila becomes more reserved and begins to cultivate a close friendship with Carmela, another neighborhood girl. Elena struggles both with the transition to middle school and eventually with the experience of puberty. Meanwhile, the neighborhood is also revitalizing and local businesses are beginning to thrive. Among the beneficiaries of this shift are the members of the Solara family, whose bar expands into a pastry shop and does very good business. The two sons of the Solara family, Marcello and Michele, attract a lot of attention in the neighborhood due to their good looks, wealth, and arrogance.
Lila, meanwhile, observes these changes and hatches a plan of her own. Her father owns a shoe repair shop, but Lila dreams of the family someday beginning to design, manufacture and sell their own shoes. Secretly, she and her brother Rino begin to design and make a sample pair of men's dress shoes.
Lila and Elena's friendship continues to be close but ambivalent during their early adolescent years. Elena is very committed to her studies and typically performs well academically, but she fears she will never have the sharp mind or charisma of Lila. A strong element of competition also becomes apparent between the girls; Lila maintains her own intellectual accomplishments, and also becomes increasingly attractive to men of all ages, both in and beyond the neighborhood.
On New Year's Eve 1958, when the girls are fourteen years old, they attend a party at the home of Stefano Carracci. The event is notable because it marks a shift in his social acceptance years after the death of his father, and because he invites many families, including the children of the man accused of the murder. The party is initially pleasant but is disrupted by a firework competition with the Solara brothers turning aggressive and escalating into gunfire. As Elena finds out later, during the party Lila also experiences a strange trance-like state in which she loses her sense of perspective and time.
A few days later, during the traditional gift-giving holiday, Rino presents the shoes that he and Lila have been working on to their father without her permission. Their father is very angry, and after the quarrel Lila appears to lose all interest in her plans for the future and becomes focused only on domestic life.
A few months later, the Solara brothers intervene in a fight between Rino and Pasquale and some young men from a wealthier neighborhood. Afterwards, Marcello Solara begins visiting the shoe store regularly, spending time with Rino. Lila is suspicious of this behavior since Marcello has previously expressed romantic feelings towards her and been rejected. Marcello has won the approval of her father and mother, to the point that when Marcello expresses support and interest in the shoe design project, they decide to see if this business can succeed. Marcello also openly expresses his desire to marry Lila, and her parents urge her to consider this offer.
Meanwhile, after completing a challenging academic year, one of Elena's teachers arranges for her to spend some time at the seaside. Elena is thrilled by the independence and peace she finds there, and her experience becomes even more interesting when she is joined by other guests: the Sarratore family. Their teenage son Nino, who attends the same school as Elena, has long been the object of her romantic interest and the two draw closer as they spend time together over the summer months. Nino, however, leaves to return to Naples in mid-August and Elena continues to spend time with his family, including his father. A few weeks later, on her fifteenth birthday, Elena receives word from Lila that she is feeling increasingly threatened by Marcello and pressure from her family. That night, after Elena has gone to bed, Donato Sarratore, Nino's father, approaches her and kisses and fondles her. He suggests that the two of them sneak off together the following day.
Confused, Elena hurries back to Naples the next morning. Lila is very relieved to see her, and it quickly becomes apparent that Lila is also being courted by Stefano Carraci. In the early autumn, Stefano proposes and Lila accepts, telling Marcello that she has no intention of marrying him. Lila now enjoys wealth, privilege, and a new social status as Stefano's fiancée. At times Elena feels alienated and jealous, but she also senses that Lila needs her support. Her own life seems to be on an upwards trajectory, as she enjoys a pleasurable relationship with Antonio, and continues strong intellectual achievements in school.
Lila is married in March of the following year, aged sixteen and a half. The wedding is a major social milestone, but reflects the various tensions and conflicts in the neighborhood. Lila herself seems uncertain and even frightened before the event. Most significantly, at the wedding reception, Lila realizes that Marcello Solara is wearing the first pair of shoes she had designed and made, which had initially been purchased by Stefano. The novel ends with on a note of high dramatic tension, to be continued by the next novel in the quartet.