In his time, the yukkuri menace overpopulated the world, flooding the surface with obnoxious, ecologically-devastating, talking manjuu.
Mankind was forced to retreat to the sea, where the yukkuri could not follow, and eke out a meager existence aboard a fleet of ships until the yukkuri destroyed themselves.
Doraemon, seeing the manjuu menace in front of his very eyes, knows that his secret mission - to warn the humans before the yukkuri became entrenched - has failed before it even began.