In the aftermath of a historic war between humans and monsters, the world now cowers under monster dominion, concealing their presence. Bokka, a curious boy, reflects on the vanished Meros Warriors who valiantly defended against demons. His encounter with Kurofune unveils the Meros' mystical abilities and the formidable I-bar machines they command. Only these warriors can perceive the elusive Melody of Oblivion, embodied by a phantom girl awaiting salvation to become humanity's savior. As Bokka unexpectedly tunes into this melody during a clash with a demon, he unravels a startling truth – he shares this rare gift. Alongside diverse allies and foes, Bokka embarks on a quest where his newfound powers reveal themselves, challenging him to combat malevolence and restore the forgotten Boukyaku no Senritsu, reviving a world eclipsed by a lost enchanting harmony.