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, then becomes new Agent 3 in the continued fight against the evil Octarians.

Trivia

 * New Agent 3 has two styles of the Hero Suit. A post on Nintendo's official website hints that they're "each suited to different situations".
 * Unlike Octo Valley and Octo Canyon, it's unclear why the player can use a non-hero weapon in this campaign.
 * In a post from Splatoon's Japanese account, an Octoling (playable) can be seen in the Hero Suit, potentially confirming that Octolings will be allowed in the single player campaign of the base game for the first time.