It's utopian post-apocalyptic. A virus has completely changed the human genome so much, evolution caused humans to be able to transform into squids. Mankind no longer exists. Humans had fallen.
But, there are some aspects that never changed. Lifestyle remains the same. Civilization works as normal, and the sun rises in the east. Everyone lives just as the humans once do. Except with a terrifying flaw.
Upon in contact with an unknown foreign ink, substances in the ink causes the newly-evolved squids to deform. It is immediately put into use in the Middle West of Asia as biological warfare. Hundreds upon thousands of squids suffered deformities due to heavy bombardment of pure black ink. To combat this, scientists have come together to devise a new ink based on the skins of individual squids. This special ink is oozed from within the squid's body, therefore it doesn't create unknown foreign substances known to cause harm.
Utilizing this special ink, biological warfare in the Middle West ended abruptly, and with it, created a new sensation of ink battles. This is the precursor to what we known as the "Splat Battle" of today.
As time went past, many countries were debating on whether to ban Splat Battles. While world leaders are in different political standings, scientists continue to study how deformities in pure black ink behaves. As it turns out, the deformities have a life of its own, being able to regenerate its body parts to regain back its original form.
Quickly, this new research swept across the globe, completely changing every squid's perspective. No later than 3 years, every squid across the world would soon learn how to regenerate, defying moral ethnics, and made changes to old testaments in religions.
Soon, young squidlings, now known as kids, learned how to play Splat Battles without fear of death. This new trend then shaped into a revolutionary sport game, now known as "Splat Battles".
With regulations and rules set in stone, Splat Battles have now been promoted as part of the Olympic Games and have now enjoyed immense success with squids, young and old.
This is the setting of Splatoon, an utopian post-apocalyptic game.