The next morning, Allie's mother announces that they are returning home to Charleston immediately, that day. Allie frantically tries to find Noah, but is forced to leave with her family. The Hamiltons send Allie to New York, where she begins attending Sarah Lawrence College. Noah, devastated by his separation from Allie, writes her one letter a day for a year without reply, as Allie's mother intercepts and hides the letters from her. Noah and Allie have no choice but to move on with their lives. Allie continues to attend school, while Noah and Fin enlist to fight in World War II. Fin is killed in battle. Allie becomes a volunteer in a hospital for wounded soldiers. There, she meets an officer named Lon Hammond, Jr. (James Marsden), a young lawyer who is handsome, sophisticated, charming and comes from old Southern money. The two eventually become engaged, to the delight of Allie's parents, although Allie sees Noah's face when Lon asks her to marry him. When Noah returns home from the war, he discovers his father has sold their home so that Noah can buy the Windsor Plantation, fulfilling his lifelong dream and keeping his promise to the departed Allie, whom by now he hasn't seen for several years. While visiting Charleston to file some paperwork, Noah witnesses Allie and Lon kissing at a restaurant, causing Noah to go a little crazy, convincing himself that if he fixes up the house, Allie will come back to him. While trying on her wedding dress, Allie is startled to read, in a newspaper and quite by accident, that Noah has completed "their" house. She faints. The picture of Noah and the house haunt and trouble her, and she acts. Telling Lon she "needs to take a break," to "take care of a few things" and "clear my head," and against her better judgment, she visits Noah in Seabrook and he, when the shock of seeing her once more subsides, invites her inside, then to join him at dinner. They eat their dinner in the very room of their first lovemaking, near the piano on which she played Chopin.
In the present, it is made clear that the elderly woman is Allie suffering from Alzheimer's disease, which has stolen her memories. "Duke," the man who reads to her, is in fact her husband and first love Noah, repeating the story of their romance. Allie recognizes neither him nor their grown children and grandchildren, who beg Noah to come home with them. He insists on staying with his true love and sweetheart Allie.