While patrolling the Octo Valley interior, Cap'n Cuttlefish and Agent 3 encounter an Octoling.
A fight ensues, with Cap'n Cuttlefish being separated from Agent 3, falling further underground along with the Octoling.
They eventually land in the Deepsea Metro, operated by a mysterious company known as Kamabo Corporation.
Cap'n Cuttlefish names the Octoling Agent 8, who has lost their memory due to the impact of the fall. They decide to work together.
Agent 8 explores the Deepsea Metro, searching for a way back aboveground. They break through various experimental facilities built by Kamabo Co.
Around the same time, Off the Hook finds Cap'n Cuttlefish's communication device. While staying in touch with the Cap'n, they utilize Marina's hacking skills and Pearl's support to assist Agent 8.
Agent 8 and Cap'n Cuttlefish make their way closer to the surface.
Along the way, Agent 3 is attacked by Kamabo Co., attempting to hijack their consciousness. After a fierce battle, Agent 3 emerges victorious.
The last one standing in their way is Commander Tartar, the mastermind behind Kamabo Co.
He attacks Agent 8 with the human-like NILS Statue but is defeated thanks to the help of Off the Hook, especially Pearl's Princess Cannon.
Having overcome a number of challenges, the group makes it back aboveground and reaches Inkopolis Square.
Octarians are appearing in greater numbers on the surface, leading to battles between Inklings and Octarians.
Splatoon 3 takes place in the Splatlands. Splatsville is undergoing a transformation as Inklings and Octarians battle it out. A fresh story of a fresh hero begins. A young hero making ends meet with the help of a Smallfry companion is recruited as the new Agent 3 of the New Squidbeak Splatoon. The struggle against the Octarian army continues!