Every night, the young Toramizu Ginta dreams of a magical world and yearns to visit it. Only his friend Koyuki believes in these dreams, yet Ginta stays positive despite the slander he faces from others. His wish is granted when a large door suddenly appears before him, summoning him to the land of MAR Heaven. In this realm, weapons known as ARMS exist. Although Ginta initially delights in discovering this magical world, he soon learns about the terrible wars that once plagued MAR Heaven and the possibility of a new war breaking out soon.