First installment of the Splatoon series, released in 2015 for the Wii U.
There are two major gameplay modes: multiplayer, where the player joins a team and must play against another team and cover the majority of the map with their team's ink while trying to maintain their own designated base by keeping it covered as well, and Hero Mode, where the player follows the story and must clear various stages while battling enemies and bosses as Agent 3 of the newly formed Squidbeak Splatoon.
This tag implicates splatoon_(series) (learn more).