It will serve as a more expanded form of a tutorial before you go online.
After the Global Testfire you get the idea that Splatoon 2 is best when playing with others. But the game will also receive a separate single player mode, which will not only allow you to play offline, but also learn more about various tricks that can be used in the multiplayer.
Below you will find some additional quotes about the story, as well as the first trailer for the single player experience:
In a shocking twist, Callie – of Squid Sisters fame – has gone missing and her right-tentacle squid Marie tasks the player to search for Callie and the Great Zapfish. Players can use different weapons in single-player mode to overcome obstacles, defeat enemies and take down towering bosses – weapons like the Roller, Slosher, Splatling or the new Dualies.