Created in a time when the Cold War was becoming hotter, Nextworld is Osamu Tezuka's parody of the tense relationship between the USA (represented as the 'Nation of Stars') and USSR (known in the work as the 'Uran Federation'). The main storyline focuses on atomic tests that create a race of mutant animals known as Fumoon, with psychic powers and intelligence beyond humans, who formulate a plan to evacuate hundreds of animals and a small group of people off the planet Earth. The reason for this is due to a large toxic cloud approaching the Earth, threatening to wipe out all life. Meanwhile, the two warring superpowers draw closer and closer to a confrontation.
The anime film (based on the manga) is similar, but omits characters from the manga. Another difference is that Kenichi (a character who also appears in the Metropolis manga and its anime adaptation) is a teenager in the film, whereas he is a child in the manga.