"Megalobox" is set in a dystopian future where society is divided by class, with the wealthy living in luxury and the poor struggling in slums. The story centers on Junk Dog, an underground fighter who participates in fixed matches to make a living. Using a makeshift exoskeleton called Gear, Junk Dog competes in boxing matches that are both brutal and thrilling. His life takes a dramatic turn when he encounters Yuri, the reigning champion of Megalonia, a prestigious boxing tournament where fighters use advanced Gear to enhance their abilities. Inspired by Yuri's skill and determination, Junk Dog decides to pursue legitimacy and enters the Megalonia tournament under the alias "Joe."

As Joe rises through the ranks, he faces numerous challenges, both in and out of the ring. His journey is marked by intense rivalries, personal growth, and the struggle to prove his worth in a system that favors the privileged. Along the way, he forms bonds with Nanbu, his coach and manager, and Sachio, a young boy who becomes his biggest supporter. Nanbu, a former boxing trainer with a troubled past, helps Joe refine his skills and navigate the complexities of the tournament. Sachio, an orphan with a knack for mechanics, provides emotional support and assists with Joe's Gear.

The narrative explores themes of perseverance, identity, and the pursuit of dreams in a world rife with inequality. Joe's fights are not just physical battles but also symbolic struggles against societal oppression and self-doubt. His rivalry with Yuri serves as a central conflict, highlighting the contrast between their backgrounds and motivations. Yuri, a disciplined and elite fighter, represents the pinnacle of the system Joe seeks to challenge, while Joe embodies the raw determination of the underdog.

As the tournament progresses, Joe's victories and setbacks reveal his resilience and commitment to forging his own path. The series culminates in a climactic showdown between Joe and Yuri, a battle that tests their limits and resolves their personal and ideological conflicts. Through its gripping narrative and well-developed characters, "Megalobox" delivers a compelling story of ambition, redemption, and the enduring spirit of competition.