The official Splatoon Twitter account posted a plot overview a short while ago. Here’s a translation:
Investigative Report from the Squid Research Lab. The protagonist, wearing the Hero Suit, travels through this underground world. The Octopus Army Corps, hoping to invade the Squid’s world, built this underground facility. Is it just me that feels the Octopus’ longing for the world above-ground from the blue sky projected onto those large monitors?
Nintendo showed a first look Splatoon’s single-player during a Nintendo Direct presentation yesterday. Along with a campaign, the game will feature an extensive multiplayer mode.
