The film opens during the events of the previous movie when Nobita Megumi Oohara is in the day before his wedding with Shizuka Yumi Kakazu, however his future self is seemingly distraught. After a scolding from his mother, present Nobita grows discouraged and questions his true identity. While searching for a place to hide his exam results, he finds an old teddy bear given by his late grandmother Nobuko Miyamoto, triggering memories of her. Nobita insists on seeing her, and Doraemon Wasabi Mizuta reluctantly agrees.
In the past, they encounter younger versions of Nobita, his friends, and family. By chance, Nobita reunites tearfully with his grandmother, Nobuko Miyamoto’s portrayal, fulfilling her wish to see him during his primary school days. She also wishes to see his future bride, and they panic when discovering that the future Nobita has apparently run away from the wedding. In the future, present Nobita fills in for his future self during the wedding but leaves midway due to speech anxiety. Desperate to find the future Nobita, they discover that the time machine has vanished. Doraemon uses the “Soul Time Machine” to transport Nobita's soul to the past, enabling them to retrieve the time machine and the “Path-Finding Stick.” By using the stick, they manage to find the future Nobita, who is hiding in the present era. He explains why and how he got to this era and laments that he could have been a better person and that he would be happier if Shizuka forgot he existed entirely.
To help him overcome his resentment, Doraemon uses the Switching Rope, a trial gadget sent to him earlier, to swap the bodies of the two Nobitas. However, a salesperson from the future then arrives telling them that if they are not switched back within one hour, their memories would be erased permanently.
In a panic, the present Nobita (in the future Nobita's body) and Doraemon search for the future Nobita (in the present Nobita's body). Future Nobita, in the meantime, encounters Shizuka but gets into a fight with older bullies. While attempting to evade them, he consistently fades in and out of consciousness, but his friends join in and eventually win, scaring the bullies off. Both Nobita's then go unconscious, worrying both his friends and Doraemon. Doraemon then uses the “Anywhere Door” to go to present Nobita's body and manages to switch both of their souls, preventing their memories from being erased. Future Nobita then finally realizes the reason why Shizuka truly likes him and decides it is time to return home but not before going to the day of his birth, where they learn about the meaning of their name.
They all return to the wedding venue, where present Nobita and Doraemon bid farewell to future Nobita. Future Nobita delivers a heartfelt speech referencing his recent experiences and notices his past self with his crying grandmother observing the wedding. He then confesses to a confused Shizuka that Doraemon has saved him for one final time.
Back during the time his grandmother is alive, she points out the strengths Nobita has and Nobita bursts into tears listening. They part ways and return to the time machine. In a pretend fight, Nobita is accidentally hit with the “Forgetting Stick,” causing him to forget most of the day's events. Doraemon decides it is better to keep it that way and assures Nobita that his grandmother's wish will be fulfilled.
During the end credits, many moments of Nobita's past and future life and the events on the current day are seen, with the final frame showing him with his grandmother and him with Shizuka.
