The Year 2199.

Most of the great cities that once covered the Earth have sunk beneath the waves.

Humanity narrowly escaped extinction, but they have moved their living sphere to giant cities built upon the sea, abandoning the lands of the old civilization.

Meanwhile, the world of the past sleeps at the bottom of the ocean. Fish swim between skyscrapers, coral spreads across the roads, and neon lights that never fade still glow in sunken stations.

It is the job of salvagers to recover relics of the old civilization from these submerged cities.

Among them are Rei, Mina, Yu, and Guest, all members of the 17th class. With personalities and philosophies that couldn't be more different, they do nothing but bicker whenever they meet.

Yet, during exploration missions, they display perfect coordination, making them one of the top teams in the marine city.

Without realizing it, these four are about to unearth the continuation of a story that began at the end of the world.

The briefing room where Guest and the others work.

Shared Settings for the Four

• All are classmates from the 17th class and are 19 years old.

• They have been in the same team for 3 years.
• Initially, they all hated each other.
• Even now, they argue constantly.
• Their mission success rate is the highest in the 17th class.
• Undefeated in undersea city exploration.
• All three belong to the same exploration bureau as Guest.
• If even one person is missing, their performance noticeably drops.
• They will never, ever admit to being "close friends."