In a remote mountain cave, a young man named Masaru Aso awakens from a four-year slumber, guided by a mysterious voice. He has no clear memory of his past, only fragments of a terrible transformation. Once a promising assistant to the brilliant geneticist Dr. Mochizuki, Masaru was forcibly subjected to a forbidden experiment: the doctor implanted grasshopper genes into his body in an attempt to create the ultimate lifeform. Horrified by what he had become, Masaru fled the laboratory and collapsed into a coma inside the cave, where he absorbed the energy of the natural world over the years. Now he has emerged as Kamen Rider ZO, a bio‑engineered warrior fused with the power of nature.

The voice that called him was a psychic warning: Dr. Mochizuki’s young son, Hiroshi, is in danger. The doctor’s ultimate creation—the Neo Lifeform known as Doras—has awakened and seeks to become a complete being. Though still bound to a life‑sustaining pool of green fluid, Doras is a cruel and calculating entity with the mind of a child and the power of a god. To force Dr. Mochizuki into performing the final surgery that will free him from his dependency, Doras sends his monster spawn—a giant spider woman and a winged bat man—to kidnap Hiroshi. Masaru must protect the boy and confront the being that was born from the same mad ambition that cursed him.

The story unfolds across a series of fierce battles. Masaru transforms into ZO when his anger at evil or his desire to protect someone he loves flares, channeling the energy of the forest into his grasshopper‑themed armored form. His only weapon is his own body: he has no external gadgets, relying instead on devastating punches, chops, and a signature high‑speed kick. A specialized motorcycle called the Z‑Bringer, which materializes from his ordinary bike when he transforms, serves as his steed and a battering ram against Doras’s minions.

Along the way, Masaru encounters others touched by his fate. Naomi, a fiery female kendo instructor and the granddaughter of Hiroshi’s grandfather, helps him come to terms with his altered body and teaches him that the most important thing in the world is the heart. Hiroshi himself, a brave and innocent boy, becomes the emotional anchor that keeps Masaru fighting despite his despair. Masaru’s lost love, Ichimi, still carries a torch for him, but his transformation forced him to leave her behind. Dr. Mochizuki, meanwhile, is revealed to be a tormented figure: obsessed with his research, he created both Masaru and Doras, yet his paternal love for Hiroshi ultimately leads him to betray his own creation.

The story reaches its climax at Dr. Mochizuki’s laboratory, where all truths are laid bare. Doras, enraged by the doctor’s refusal to perform the final surgery and by his affection for Hiroshi, drags the doctor into his life‑pool and merges with him. He then absorbs Masaru himself, transforming into a monstrous red‑skinned form far more powerful than before. But Hiroshi’s desperate cries and the melody of a fallen music box overwhelm Doras’s mind, giving Masaru the opening to break free. Using the full force of nature’s energy, Masaru delivers a soaring ZO Kick that shatters Doras’s body. With his creator gone and his own form destroyed, the Neo Lifeform fades, crying out for a father who never loved him as he wanted.

After the battle, Masaru stands alone again. He has regained his humanity, but his life is forever changed. The manga closes on a note of quiet resolve: a lonely hero who must go on protecting the world, carrying the sorrow and the strength of the natural world within him.