Because of global warming, glaciers around the world are melting rapidly. This is causing not only rising sea levels, but also the emergence of ancient creatures sealed beneath the glaciers in the blue abyss, known as sea demons.
To fight the sea demons, every nation in the world joined forces to build a maritime Great Wall and invent giant combat robots, codenamed "Giants." Research into ancient ruins and the sea demons also led to the discovery of a new energy source, which they harnessed for battle and called magic.
As the war against the sea demons continued, humanity gradually turned its attention to the blue abyss. Relying on the "Giants" and magic, they set out to uncover its secrets and save humanity…
