Octo Valley (also known as Hero Mode) is a singleplayer game mode in Splatoon. It involves the player donning the Hero Suit and venturing into a hidden, underground valley where the Octarians dwell, to rescue Inkopolis' stolen Zapfish and defeat the Octarian menace.

Story - After the Great Zapfish, Inkopolis' primary power supply, is stolen, Cap'n Cuttlefish employs a Splatoon of Inklings to travel to the Octarians' secret underground base and retrieve it. Players assume the role of Agent 3 and traverse the treacherous landscape of Octo Valley in order to reclaim various lost Zapfish, Sunken Scrolls, and Power Eggs.

Sunken Scrolls - Sunken Scrolls are rolled up pieces of paper that can be found in Octo Valley. They are long-lost drawings and other pieces of information that depict the history and lore of Inkopolis. One Sunken Scroll can be found in each level of Octo Valley. The scrolls in the Boss Kettles earn you a new weapon for the online battles.

Rewards - Playing through Octo Valley is not without its rewards. For defeating the final boss, the player is awarded a full Replica of the Hero Suit and a set of Octoling Gear. Also, the Sunken Scrolls found in each boss stage will unlock further weapons in Ammo Knights that are available for purchase once the appropriate level requirement is met.