"Megaton-kyū Musashi" is set in a post-apocalyptic world where humanity faces extinction due to an alien invasion by a mysterious race known as the "Dracters." The story begins in a seemingly peaceful town where the protagonist, Yamato Ichidaiji, lives an ordinary life. However, this tranquility is shattered when the Dracters attack, revealing that the town is actually a simulated reality created to protect humanity. Yamato and his friends, including Ryugo Hijikata and Teru Asami, are recruited to pilot giant mechs called "Rogue" to combat the alien threat.

The narrative unfolds as the trio discovers the truth about their world and their own identities. They learn that they are part of a select group of individuals chosen to fight the Dracters, who have already decimated much of Earth. The mechs they pilot, known as "Musashi," are humanity's last hope, combining advanced technology with the pilots' unique abilities. As the story progresses, the characters face numerous challenges, both on the battlefield and within their own ranks, as they uncover secrets about the Dracters and the true nature of their mission.

The series explores themes of survival, identity, and the resilience of the human spirit. The characters must grapple with their pasts, their relationships, and the moral complexities of their mission. Yamato, in particular, undergoes significant development as he transitions from a carefree teenager to a determined leader. The plot is driven by intense mech battles, strategic alliances, and the gradual unraveling of the mysteries surrounding the Dracters and the simulated world.

Supporting characters, such as the enigmatic scientist Dr. Minato and the skilled pilot Koyomi, add depth to the narrative, providing both assistance and conflict as the story progresses. The series balances action-packed sequences with moments of introspection, creating a dynamic and engaging storyline. As the characters fight to reclaim Earth, they must also confront the ethical dilemmas of their actions and the sacrifices required to ensure humanity's survival.