On this day, a new amusement park called "20th Century Expo" suddenly opened near Kasugabe where Shin-chan lived. The amusement park is full of old TV shows and movies from the 20th century. Here, everything is a replay of the past. Every adult who comes here is immersed in the old times, but for children, these things are meaningless. What Shin-chan never expected is that the parents who entered the amusement park ignored other things and just indulged in the past. All the adults in the city went in, leaving only the children! Whose conspiracy is this? Can Shin-chan and his friends crush this conspiracy?