Gist of the story..

doraemon's battery dried up and there wasn't any way to repair him cos of the complex technology that was used to create him..

nobita has two choices- to forcefully change the battery and lost all of doraemon's memories, or wait till there is a solution.

Nobita doesn't want the precious memories to be deleted. So he became very determined and studied hard to find a solution to replace doraemon's batteries. Finally, 35 years later, he grew up, married his childhood sweetheart and found a way to restore doraemon's memories..