At a modern-day nursing home, an elderly man, Duke, reads a romantic story from his notebook to a fellow patient.
In 1940s Seabrook Island, South Carolina, Noah Calhoun is smitten with seventeen-year-old heiress Allie Hamilton after seeing her at a carnival, and they have a summer love affair. Noah takes Allie to an abandoned house that he intends to buy for them. They try to make love, but are interrupted by Noah’s friend Fin with the news that Allie’s parents have the police looking for her.
When Allie and Noah return to her parents’ mansion, Allie’s mother Ann calls Noah trash and they ban her from seeing Noah. Noah walks out and Allie chases after him. The ensuing argument between the two ends in a break up and the next morning, Ann announces that the family is returning home to Charleston. Allie attempts to contact Noah, but is unable to find him, so she asks Fin to tell Noah that she loves him. When Noah gets the message he rushes to Allie’s home, only to find the house empty.
Noah writes to Allie every day for a year but Allie’s mother intercepts the letters and they never reach Allie. Noah enlists with Fin to fight in World War II, where Fin is killed in battle. Allie volunteers in a hospital for wounded soldiers, where she meets officer Lon Hammond Jr., a young lawyer who comes from old Southern money. The two become engaged, to the delight of Allie’s parents.
When Noah returns from the war, his father has sold their home so that Noah can buy the abandoned house. While visiting Charleston, Noah witnesses Allie and Lon kissing at a restaurant; he convinces himself that if he restores the house, Allie will come back to him. Allie is startled to read in the newspaper that Noah has completed the house to the specifications she made years before.
Allie returns to Seabrook to find Noah living in the restored house. The two renew their relationship and make love. In the morning, Ann appears on Noah’s doorstep, warning Allie that Lon has followed her to Seabrook. Ann reveals that in her youth she had been in love with a lower-class young man and still thinks of him. He admits that he has often thought of her as well. She then gives Allie the letters that Noah wrote to her, admitting that she hid them from Allie. Allie confesses to Lon that she has been spending time with Noah, and tells him she knows she should be with him, but she remains indecisive. Ultimately, she goes back to her relationship with Noah.
In the present, it is revealed that the elderly woman is Allie, who is suffering from dementia. Duke is actually Noah, and was told by Allie during the onset of her illness to reread their journals to help her remember. She briefly remembers who he is and they reconcile, but soon forgets and panics, forcing medical personnel to sedate her. Noah has a heart attack and is sent to the hospital while Allie is sent to a dementia ward in the hospital. He visits her room in the middle of the night, and she remembers him again, expressing that she may forget again. Noah reassures her that they can do anything with the love they share, and fall asleep together in Allie’s bed, dying in their sleep with a nurse discovering them in the morning.