Nobi Nobita is a fourth grader who constantly gets failing grades in his subjects due to his laziness and is always bullied by his classmates Honekawa Suneo and Gouda Takeshi. His great-great grandson from the 22nd century, Sewashi, who watches him every day, travels to Nobita's timeline while bringing along his robotic cat Doraemon. Sewashi reveals that if Nobita keeps up his act, he will have a disastrous future: he will marry Takeshi's sister Jaiko, have his private company burned down, and will be left with a great debt. To circumvent this, he orders Doraemon to help Nobita and activates a program to prevent Doraemon from returning to the 22nd century unless Nobita gains a better future.

Source: Wikipedia

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Parent story: Doraemon (1979)
Alternative version: Kaette Kita Doraemon
Alternative version: Nobita no Kekkon Zen'ya: The Night Before a Wedding
Sequel: Stand By Me Doraemon 2