In Japan, adults become enamored with 20th Century Expo, nostalgic theme parks recreating the 1970s heyday. When they vanish into these parks, their children are left behind. This disappearance is orchestrated by 'Yesterday Once More,' a group longing for the past and aiming to revert Japan to the 20th century's ideals. Shinnosuke, along with his parents Hiroshi and Misae, battles against 'Yesterday Once More' to preserve their family and embrace the 21st century.