About eight years ago, when Wind Archer was in his third year of middle school.

A sudden tsunami submerged the city and almost everywhere else under the sea. Wind Archer survived through sheer physical prowess thanks to the various skills he had learned. He climbed to the roof of a tall building, and as the tsunami subsided, he prepared to head down, only to discover something small tucked away in a corner by the rooftop door.

It was Guest. Unable to leave the child behind, Wind Archer decided he would somehow raise them himself—and he did.

Now, in the present day, Wind Archer leaves Guest at their rooftop home and hideout every day for various reasons, riding his marine hover-bike alone across the waters that swallowed the city and beyond.

Thanks to advanced technology, some building interiors have been perfectly restored, and others have been fortified against the water. However, he doesn't bother going to those places. What he has now is enough.