Masaru Aso was the lab assistant of geneticist Doctor Mochizuki, and was used as one of his experiments related the creation of the Neo Organism, enabling him to transform into a grasshopper-like being called Kamen Rider ZO. He fled into the mountainside and went into a coma before he awoke by a telepathic call two years later with an unconscious urge to protect Hiroshi Mochizuki, the son of Doctor Mochizuki. After an attempt to uncover the meaning of his transformation at Mochizuki Genetics, Masaru senses Hiroshi in danger and saves the boy from Doras as ZO. Masaru learns that Dr. Mochizuki mysteriously vanished before he revealed himself after a misunderstanding with Reiko and her karate class. By then, ZO battles Koumori Man to cover Hiroshi and Reiko's escape before they are sucked in a pocket dimension by Kumo Woman, both monsters created by Doras. But ZO saves them and kills Kumo Woman before Koumori Man swoops down and snatches Hiroshi off, with ZO in pursuit. After saving Hiroshi, Masaru reveals to Seikichi that Dr. Mochizuki used him in his experiments. Refusing to believe it, Hiroshi ran off before Masaru found him and fixed the watch, recognizing the melody that stirred him out of his rest as he helps Hiroshi cope with this new information. However, Koumori Man assumed Mochizuki's form to lure Hiroshi away and capture him with Doras knocking Masaru out cold. Making his way to a complex, ZO kills Koumori Man before making his way to Hiroshi and Dr. Mochizuki, learning that the geneticist was the one who woke him up and that the Neo Organism has been acting on its own whim the entire time to become the perfect being. ZO attempts to fight Doras, only to be assimilated into the Neo Organism. Doras then proceeds to use the boy to force Mochizuki to complete its evolution. However, the watch manages to hold Doras at bay as ZO breaks out of the monster and Mochizuki sacrifices himself to destroy the pool, the Neo Organism's life source. The complex then undergoes a detonation sequence as ZO and Hiroshi escaped with their lives. Dropping Hiroshi with Seikichi, Masaru leaves to parts unknown.
