Masaru Aso is the lab assistant to Doctor Mochizuki, a geneticist pursuing experiments tied to a mysterious Neo-Lifeform. When the research pushes Masaru further than expected, he undergoes a dramatic change and becomes the grasshopper-like Kamen Rider ZO. He flees into the mountains and slips into a two-year coma, only to be awakened by a telepathic call that orders him to guard Hiroshi Mochizuki Hiroshi Mochizuki, the scientist’s son. Fueled by this strange summons, Masaru senses that Hiroshi is in danger and steps in to protect him from the threat of Doras Doras as the transformed hero.
Doras proves to be a formidable foe, regenerating after every clash and giving birth to two new threats that pursue Hiroshi. Meanwhile, Hiroshi’s life is increasingly at risk, and he seeks help from his eccentric inventor grandfather, Seikichi. Seikichi is skeptical at first, brushing off Hiroshi’s monsters tale, but his stance shifts when Masaru—still in his ZO form—appears outside Seikichi’s home and intervenes to keep Hiroshi safe.
The drama intensifies when Hiroshi crosses paths with Reiko Reiko and her karate class at their dojo. Masaru reveals himself as Hiroshi’s bodyguard, a revelation that adds tension to the already perilous situation. A terrifying bat-based assault follows, and a student knocks the flying monster from the air, revealing a humanoid bat known as Bat-Man. Masaru, in his ZO guise, engages Bat-Man to cover Hiroshi and Reiko’s escape, but the danger escalates when the duo is pulled into a pocket dimension by another Doras creation, Spider-Woman. ZO returns to rescue them and obliterates Spider-Woman, though Bat-Man renews his kidnapping attempt.
After this rescue, Masaru confides in Seikichi about Hiroshi’s father’s involvement in the experiments. Hiroshi resists the claim at first, insisting it cannot be true, and Masaru works to reassure him by repairing a keepsake watch and recognizing the melody that triggered his awakening as a sign of the truth. Just when Hiroshi begins to accept the possibility, he spots his father alive in a crowded city. Confronting the man, Hiroshi learns that the Bat-Man persona is actually his father in disguise.
Masaru encounters a full-powered Doras in a brutal showdown, taking a hard fall that sends him crashing onto a car after being hurled from a building. He regains consciousness when a grasshopper guides him to Hiroshi’s location and leads him toward a hidden complex. There, ZO faces Bat-Man once more and defeats him. He also discovers that the Neo-Lifeform has been acting with its own will, aiming to use Hiroshi’s father’s research to perfect itself. The Neo-Lifeform materializes from a strange pool and speaks in Hiroshi’s voice, outlining its plan to complete the work through Hiroshi’s participation, then merges with Doras to become the formidable “Red Doras.”
In a pivotal moment, the sound of a melody from Hiroshi’s treasured watch disrupts Doras long enough for Professor Mochizuki to sacrifice himself, dealing a critical blow to the Neo-Lifeform’s pool and allowing ZO to escape a forced fusion. With a final Rider Kick, Kamen Rider ZO defeats Doras and destroys the Neo-Lifeform. The complex begins to self-destruct, and Masaru carries Hiroshi to safety, escaping the flames with the help of Seikichi. In the end, Hiroshi and Masaru part ways with a sense of loss and a stubborn hope for the future, while Masaru heads toward an uncertain horizon.
