After fulfilling the praying ceremony, the three oldest children of the alien Fog Mother hunt an ideal human, in order to feed that person to their newly hatched siblings. They find such a person in Kana, the young friend of environmentalist reporter Kouji Segawa as he was investigating the pollution by the lake. Protecting her sister from the fiends as they escape them, Kouji is thrown over the cliff by the reptilian Agito as Kana is taken by the insectoid Zu. However, Kouji was resurrected by the Earth Spirits as a J-Powered Warrior, Kamen Rider J, acting on their will to fight the Fog with their emissary Berry as his guide. With Fog Mother sensing Kouji's presence, Agito attempts to finish the job in his true form, only to be killed by J. Hurrying to Kana, Kouji enters their domain and faces Zu as she assumes her true form. Facing her, J manages to mortally wound Zu as she grabs him, taking to before they crash into the fortress which is Fog Mother. After she dies, Kouji learns of Fog's intention of feeding humanity to as Garai completes the ritual and sends Kana to the hatching chamber while manhandling Kouji. Berry intervenes to free Kana from Fog's spell before being struck down by Garai as Kana is sent down to her death while Fog Mother begins attacking a nearby city to prepare for her children's awakening. Assuming his Rider form, J fights Garai in his true form, Cobra Man, in a heated battle. After killing Garai, J is being digested by Fog Mother as he attempts to save Kana. However, absorbing the life energy around him, J kills off Fog's newly born spawn as he escapes the bowels of Fog Mother and assumes Jumbo Formation. After ripping Kana out of Fog Mother, J battles the monster before killing her as she attempted to take him down with her. Escaping Fog Mother's destruction, Kouji takes Kana to a serene environment with Berry watching from a distance.
Fog (フォッグ,Foggu?) are a group of monsters whose very presence is signaled by a mass amount of mist. Every millennium, the Fog Mother gives birth to a new brood to feed on the occupants of the world she landed on. One such brood was responsible for the mass extinction of the dinosaurs, which were their sole food supply. Seventy five million years later, the Fog Mother returns to Earth with a new brood while her three children perform the ritualistic prayer before going out to obtain the sacrifice for their siblings to feed on so they may consume the human race.