An ancient alien parasite known as the Fog Mother, which previously caused the mass extinction of the dinosaurs, returns to Earth after 75 million years to hatch a new brood that will consume all human life. Environmental photographer Kouji Segawa is investigating the mysterious deaths of animals and trees in a mountainous region when he encounters a young girl named Kana Kimura. The Fog Mother’s three eldest children, who can assume human forms, capture Kana to serve as a sacrifice for their unborn siblings. When Kouji tries to protect her, the reptilian monster Agito throws him off a cliff to his death.

Kouji’s selfless act impresses the Earth Spirits, mysterious beings who reside within the planet. They revive him and grant him the power of J-Spirit, allowing him to transform into the insectile warrior Kamen Rider J. A large, talking grasshopper named Berry serves as his guide from the Earth Spirits. With his new abilities and his transformed motorcycle, the J-Crosser, Kouji battles the Fog Mother’s children. He first destroys Agito and then confronts the wasp-like monster Zu, killing her as she crashes into the Fog Mother’s golden battle fortress.

Kouji learns that the eldest sibling, Garai, is completing a ritual to send Kana to the hatching chamber. After Berry is struck down trying to save the girl, Kouji battles Garai in his true form, the cobra-like Cobra Man, and defeats him. To rescue Kana, Kouji is swallowed by the gigantic Fog Mother, who begins digesting him. Absorbing the life energy from his surroundings, he fights his way out, destroys the newborn offspring, and triggers his greatest ability: the Jumbo Formation, which grows his body to the height of a giant, roughly 40 meters tall. In this giant form, Kouji tears Kana from the Fog Mother’s body and destroys the alien parasite with a devastating Jumbo Rider Kick. Following the battle, Kouji, freed from his duties, brings Kana to a peaceful place while Berry watches from afar.