孫悟空は存在すると信じているのび太は、タイムマシンで、1300年前の唐の時代の中国へ。そこで自分そっくりの孫悟空に出会い、大喜び。みんなを連れ、再び唐の時代へ向かうが、いつまで経っても孫悟空は現れない。しかたなく、ヒーローマシンで孫悟空になりすましたものの、すぐにばれてしまう。
現代に帰ってみると、ママの頭に角が！ 知らない間にヒーローマシンから妖怪が飛び出し、地球を妖怪の世界にしてしまったのだ…！
Nobita, Gian, Shizuka, and Suneo is preparing for a school play on the "Journey to the West". As they are arguing about who should play the role of the monkey king, Nobita suggested that the real monkey king should play the role. Because the monkey king is only a legend, Nobita and Doraemon have decided to go back in time using the time machine and make a fake one and show Gian, Shizuka, and Suneo to prove that they are right. Unfortunately, Doraemon's machine allowed the fictional monsters to come to the real world and defeated the entire human races, thus turning the future (Nobita's time) into a demon-ruled world. In order to reverse the change, Doraemon and gang needed to return to the past and capture the demons back into the machine. On the way, they have met the real monk and rinrei (a child). At the end, they have either returned the demons back to the machine or destroy them with Dorami's help, thus turning the future back to normal.
