Return of the Mammalians

Return of the Mammalians is a single-player game mode in Splatoon 3. It involves the player donning a Hero Suit and venturing into Alterna (the setting of the story) to defeat the Octarian army.

Story
An Inkling and their little buddy, a Smallfry, are scrap hunters who search for junk gems by day and splat it up in Turf Wars on the side. This Inkling meets the New Squidbeak Splatoon and is now their new Agent 3 in the continued fight against the evil Octarians.

Trivia

 * New Agent 3 has 2 styles of the Hero Suit. A post on Nintendo's official website hints that they're "each suited to different situations".