250 years ago, Earth's sea level started to drastically rise...

Dreadful monsters from the sea came and attacked humans,

The land was gradually but certainly becoming the sea.

Those monsters became known as Abyssal Fleet.

Humans succeeded in capturing the monsters.

To their surprise, the monster's organism are close to human's.

Humans develop the Anti-Abyssal Fleet weapons by improving the monster.

Somehow, only female beings could be produced, as they reject sex chromosomes that would determine maleness.

Thus...