Agent 8 wanted to escape the surface but they were forced into a test area which is basically the subway named "Deepsea Metro".
It turns out that the events took place after Splatoon 1's story mode and that Captain Cuttlefish and Agent 3 were separated after the fight with Agent 8 which took place probably after DJ Octavio confrontation.
The song that Callie and Marie sang during the fight (Calamari Inkantation) affected the minds of the octarians, freeing them from DJ Octavio's control. Agent 8 is one of the octarians.
That song drives Agent 8 to reach the surface and Inkopolis. The song made a huge impact on them.
In order to reach the surface, Agent 8 and Cuttlefish had to find their way out. On the way, there's a telephone that mentions the tests, the "promised land", and the four "thangs". Agent 8 and Cuttlefish proceeded to enter the Deepsea Metro and the tests while meeting Pearl and Marina via long distance communication from the surface to underground.
The test's rewards are Mem Cakes which are supposedly Agent 8's memories.
After collecting the mem cakes and the four thangs (which formed a blender), the telephone reveals itself to be an ai from 12,000 years ago named Commander Tartar and proceeded to betray both Agent 8 and Cuttlefish by blending them into paste only to be stopped by Agent 3.
Tartar now controls Agent 3 and both Agent 8 with Cuttlefish proceeded to follow them to the surface which both Agent 3 and 8 fight once again. After the fight, Commander Tartar then uses the huge statue like machine to destroy Inkopolis. Pearl and Marina then came up with the plan to ink the entire statue which they assigned agent 8 to do.
After Agent 8 covers the statue with ink, Pearl then deals the statue with the finishing blow which is the Killer Wail. With the statue and Tartar now destroyed. Agent 8 went to the surface safely along with Pearl, Marina, Agent 3, and Cuttlefish.
It was unknown how Inkopolis Square got bleached but we'll find out once Side Order releases.
