The player spawns on a small island in the middle of a canyon (hence the name) and are greeted by the (retired?) Squid Sister Marie. She takes on Captain Cuttlefish's role and recruits you as Agent 4 of the New Squidbeak Splatoon.
Throughout Octo Canyon you will search for invisible kettles and ink them to go into levels. After completing a certain amount of levels in a world you are presented with a boss kettle, where you will attempt to defeat a boss with three health levels.