While performing in a student film, Ataru cuts down Tarozakura, a large, ancient cherry tree. Strange things begin to happen all across Tomobiki: a mountain appears out of nowhere, spring changes to winter, and Lum loses her powers, while those around her act as if she doesn't exist.

45 years ago, a man was subjected to the experiments of the secret organization Shocker and became a cyborg. Since that day, the man's sole mission has been protecting humanity and justice from the threat of Shocker. His name is Takeshi Hongo, the first Kamen Rider. After a long battle overseas, Hongo hears a certain girl is in danger and immediately returns to Japan. He then meets Takeru Tenkuji, Kamen Rider Ghost, who had been investigating the girl's whereabouts.

When local teacher Midori Yamabuki receives an emergency summons to her home planet it turns out to be a trap devised by galactic warlord Dr. Mashirito, and it's up to Dr. Senbei Norimaki and a group of students (including his robot daughter Arale) to venture into space to save her.