The disappearance of her old friend, Lila Cerrullo, causes 60-year-old author Elena Greco to reflect on the early days of their friendship in 1950s Naples, when Elena and Lila are two very different girls at the top of their class. When the girls discover the lair of Don Achille, a criminal figure who controls the neighborhood, they decide to confront him.

On the island of Ischia, Elena runs into the Sarratore family, reigniting her childhood crush on Nino, who is at odds with his father, the charming Donato. Back in Naples, Lila's resistance to Marcello's pressing courtship reaches a breaking point. On the day of Elena's 15th birthday, an unexpected encounter leaves her shaken.

Elena returns to Naples to find Lila has made a romantic connection with Stefano, despite the looming presence of Marcello. As Lila embraces a more glamorous life at Stefano's side, Elena warns her about what people are saying around the neighborhood. Later, in hopes of keeping up with her friend, Elena encourages one suitor and rebuffs the advances of another.

At Rino and Pinuccia’s wedding, Elena accepts Lila’s invitation to go on holiday, provided the destination is Ischia, where she knows she’ll find Nino. On the island, Elena, Lila and Pinuccia begin to spend more and more time with Nino and his friend Bruno and Lila starts to take a strong interest in Elena’s summer reads. As Lila and Pinuccia exhibit signs of nervousness when their respective husbands come to visit for the weekend, Elena is thrown when Lila comes to her bedroom to make a confession.

As Pinuccia returns to Naples, a new secret excites Lila and numbs Elena, who nonetheless becomes embroiled in its coverup. Subdued by Lila's charisma and indifferent to Bruno's flattery, Elena finds herself the keeper of a far darker clandestine series of events involving Donato Sarratore. After the girls encounter Michele and Gigliola, who have come to Ischia on a day trip, Lila seizes the opportunity to throw the whole truth in Stefano's face.

On a visit to the neighborhood for the first time in years, Elena attempts to make peace with Immacolata and learns that her mother has been unwell. Later, when Elena and Nino return from a trip to New York, she and Lila grow closer after revealing that they’re both pregnant.