Planet Harmony, Arnold's Sanctuary. Planet Harmony is a place for all characters from all Games, TV Shows, Anime & etc., to see Planet Harmony as their home. But on May 5th 2042, Corona Virusknife, Emperor's Older Sister, took over Planet Harmony. Her goal was to eradicate the entire population of Planet Earth, Arnold's home. Arnold didn't know that his Sanctuary; Planet Harmony was fully taken over by Corona. What's even worse, Emperor, his Inkling son vanished after he committed one of the worst revenge moments of Planet Harmony silencing the entire community of Inkopolis. Also, the disappearance of the Squid Sisters' Callie & Marie, Off the Hook's Pearl & Marina and Deep Cut's Frye, Shiver & Big Man after Emperor silenced the Splatoon Community. Inkopolis (Combined Cities of Inkopolis Plaza, Inkopolis Square & Splatsville) was once a vibrant city where all Inklings, Octolings, Salmonlings & other races interacted with each other and went to stores to get their fresh drip. Now Inkopolis was silenced by Emperor and his friend Solaris and destroyed by a bomb known as the Killswitch.
Now it's Arnold's job was to free from Corona's tyrannical grasp and restore order to the Sanctuary once again.