In the year 3990, the immortal Oldna Poseidal rules Pentagona, a war-torn solar system comprising five planets. Daba Myroad is a survivor of the Yaman Clan, one of many native societies nearly exterminated by the tyrant. Residing on the remote planet Koam with his friend Mirao Kyao, Daba possesses L-Gaim, a humanoid mecha commonly referred to as a "Heavy Metal" and the last known relic of the Yaman Clan. While fleeing from a band of thieves attempting to steal L-Gaim, the pair befriend Fanneria Amu, an aspiring actress, and Lilith Fau, the last surviving fairy in Pentagona. Daba promises to fulfill the dying wish of one of the thieves hunting him—an honorable act that leads him to the powerful merchant Amandara Kamandara. With their mission in sight, the ragtag group will face powerful adversaries and become entangled in a rebellion against Poseidal's reign.