Several years after being subjugated to an inhumane experiment, Masaru Asou wakes up in a forest, hearing the cries of an unknown voice to protect Hiroshi Mochizuki, the son of the scientist who experimented on him. Hiroshi's life is endangered by a humanoid monster, but is saved by Masaru, who becomes the cyborg Kamen Rider ZO. Masaru then realizes that Professor Mochizuki has gone missing for the past two years and the "Neo-Organism" experiment he was working on had cultivated to the monster that attacked Hiroshi. But when Hiroshi is abducted by the Neo-Organism, Kamen Rider ZO rushes to save him and find out the true whereabouts of Professor Mochizuki.