"Junk: Record of the Last Hero" is a science fiction manga series set in a dystopian future where humanity faces extinction due to the emergence of monstrous creatures known as "Hazards." These creatures, born from human experimentation with genetic engineering, have overrun the planet, leaving only a handful of survivors. The story centers on Junk, a young man who becomes humanity's last hope after undergoing a transformation that grants him extraordinary abilities.

Junk begins as an ordinary individual living in a fortified city, one of the last bastions of human civilization. After a tragic encounter with Hazards, he is critically injured and subsequently infused with Hazard DNA by a scientist named Dr. Kales. This procedure not only saves his life but also grants him superhuman strength, regenerative abilities, and the capacity to transform into a powerful, monstrous form. However, this transformation comes at a cost: Junk must constantly battle the Hazard DNA within him, which threatens to consume his humanity.

As Junk grapples with his new identity, he joins forces with other survivors, including a skilled fighter named Sara and a mysterious girl named Miu, who harbors her own secrets. Together, they embark on a journey to confront the Hazards and uncover the truth behind their creation. Along the way, they encounter various factions, including rogue scientists, military remnants, and other transformed humans, each with their own agendas.

The narrative explores themes of survival, identity, and the ethical implications of genetic manipulation. Junk's internal struggle between his human side and his Hazard instincts serves as a central conflict, driving much of the character development. The series also delves into the origins of the Hazards, revealing a complex web of human ambition and hubris that led to the world's downfall.

As the story progresses, Junk and his allies face increasingly powerful Hazards and uncover deeper layers of conspiracy. The stakes escalate as they learn that the Hazards are not merely mindless monsters but part of a larger plan that could determine the fate of humanity. The series builds toward a climactic confrontation that tests Junk's resolve and forces him to confront the ultimate question of what it means to be human in a world where the line between man and monster has blurred.