Ginta is a Japanese high school boy who often fantasizes about a land known as Mar Heaven. One day, he is mysteriously summoned to the imaginary land, which in fact turned out to be real. Upon reaching there, he learns of powerful weapons known as Arms that exist in Mar Heaven. With the companions and friends that he meets during his travels, the group seeks to destroy the Chess, a faction led by the mysterious Phantom, which is determined to bring about Mar Heaven's Ultimate Demise for reasons unknown.