At a modern-day nursing home, an elderly man, Duke, reads a romantic story from his notebook to a fellow patient.
In 1940, Seabrook Island, South Carolina, poor lumbermill worker Noah Calhoun sees 17-year-old heiress Allison “Allie” Hamilton at a carnival, pursues her, and they have a summer romance. After weeks of dating, Noah takes Allie to an abandoned house that he intends to buy for them. They try to have sex for the first time, but are interrupted by Noah’s friend Fin with the news that Allie’s parents have the police looking for her. Due to their difference in social class, Allie’s parents were never fond of Noah, and were betting against the relationship from the beginning.
When Allie and Noah returned to her parents’ mansion, Allie’s mother Anne ruthlessly insulted Noah solely due to his social status and banned her from seeing him. Noah walks out and Allie chases after him; an argument ensues, leading Allie to break up with Noah in the heat of the moment. The next morning, Anne announces that the family is returning home to Charleston that same day. Allie attempts to contact Noah, but is unable to find him, so she asks Fin to tell Noah that she loves him. When Noah receives the message he rushes to Allie’s home, only to find the house gated up and empty.
Noah wrote a letter to Allie every day for a year, but it is later revealed that Allie’s mother had been intercepting the letters so that they never reach Allie. After 365 letters, Noah gives up and stops writing to the woman he’s still madly in love with. While apart, Noah enlists with Fin to fight in World War II, where Fin is killed in battle. Allie volunteers as a nurse’s aide in a hospital for wounded soldiers, where she meets captain Lon Hammond Jr., a young lawyer who comes from old Southern money. After a few years being together, 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 stumbles upon seeing Allie and chases after her only to witness Allie and Lon kissing at a restaurant; he convinces himself that if he refurbishes the house, Allie will come back to him. After not seeing or hearing from Noah in years, while Allie is being fitted for her wedding dress, she is startled and faints when she sees in the newspaper that Noah has completed the house to the specifications she made years before, and is trying to sell it.
Allie is overwhelmed with memories and unresolved feelings for Noah, and asks permission from Lon to take a little trip before the wedding. She returns to Seabrook to find Noah living in their dream house. The two rekindle their relationship and eventually have sex. Several days later, Anne appears on Noah’s doorstep to warn Allie that Lon has followed her to Seabrook. Anne reveals that, in her youth, she had been in love with a lower-class young man and was still thinking about him; Anne then gives Allie the letters that Noah wrote to her as an admission that she had hidden them from Allie. Subsequently, Allie confesses to Lon that she had spent time with Noah, and eventually returns to her relationship with Noah.
In the present narrative, it is revealed that the elderly woman is actually Allie now stricken with dementia and Duke is actually her husband Noah. Years ago during the early stages of her illness, he had been told by Allie to re-read their journals to help her recall her past. One day during the reading sessions, she briefly remembers Noah and wants them to run away together. He reminds her of her dementia and she asks how long they have before she forgets again, and Noah tells her that the last time it happened, it was about five minutes.
She cries and they embrace, but Allie soon forgets who Noah is again and in her ensuing panic, medical personnel are forced to sedate her. Noah suffers a heart attack and is sent to the hospital, while Allie is sent to a dementia ward in the same hospital. Noah visits Allie’s room at midnight, causing her to remember him again. He proceeds to remind her of her illness again, though he reassures her that they can do anything with the love they share. They kiss, hold hands, and fall asleep together, with Noah telling Allie, “I’ll be seeing you”. In the morning, a nurse discovers that they have both died in their sleep, with their hands still clasped together.