89 years ago, something hit the moon, pushing it closer to the earth. A huge red block showed up on the moon, and a tower connected the earth to the red block. Most lands on earth are covered by ocean, and the only way for humans to live is to move into the tower.

They named the red block "Red Door" and the tower "Arbor."

At the present time, a maintainer named Xylia gets hit by a piece of fragments. After she wakes up again, she begins to see weird things. The doctor tells her the only way to solve her issue is to help them discover the truth about the Red Door.