The central twist of "The Notebook" is the revelation that the elderly couple in the nursing home, Duke (James Garner) and the patient (Gena Rowlands), are in fact the older versions of Noah and Allie. The love story Duke has been reading is not fictional; it is their own life story, which he reads from a notebook Allie wrote years before when she was diagnosed with dementia. She instructed him to "read this to me, and I'll come back to you." Noah, using the pseudonym Duke to avoid startling her, reads to her every day in the hope of sparking her memory.
The narrative structure, which alternates between the past and present, masterfully conceals this connection until the climax. In the film's final act, Allie has a brief, miraculous moment of lucidity where she recognizes Noah and remembers their life together. They share a tender dance and a moment of connection before her dementia returns, causing her to panic at the sight of a "stranger" (Noah). This emotional turmoil causes Noah to suffer a heart attack. The ending reveals the ultimate depth of their bond: after recovering, Noah sneaks into Allie's room one last time. She recognizes him again, and they fall asleep holding hands. The next morning, a nurse discovers they have both passed away peacefully in the night together, a final testament to their promise never to leave one another.