What single-player campaign title belongs to the first Splatoon game?

The first Splatoon’s single-player campaign is titled Octo Valley.

In Octo Valley, players control Agent 3, an Inkling recruited by Cap’n Cuttlefish to help rescue Inkopolis’s stolen Great Zapfish. The campaign takes place across underground Octarian territory and combines platforming, exploration, puzzles, and combat with the game’s signature ink mechanics.

Octo Valley is part of the original 2015 Wii U game, not a later expansion. Octo Expansion belongs to Splatoon 2 and focuses on Agent 8, while Side Order is downloadable content for Splatoon 3. Return of the Mammalians is the single-player story campaign in Splatoon 3, making all three alternatives refer to later content.

The campaign also introduces much of the series’ wider story. Cap’n Cuttlefish explains the conflict between Inklings and Octarians, a rivalry rooted in the Great Turf War. As Agent 3 progresses, the player learns more about the world while facing Octarian enemies and eventually confronting DJ Octavio.