Ritsuka Tachibana. She lives a calm and unchanging life. Ritsuka's older brother Lindo returns from studying abroad in the United Kingdom. Shikou Academy student council president Rem Kaginuki calls on the girl, and she begins having mysterious encounters with the student council members. Her life changed a little from that day, but Ritsuka expected nothing more. However, she comes home to find her mother collapsed with suspicious hooded men nearby. They were not human but devils, vampires, and exorcists seeking the Forbidden Grimoire. Ritsuka's former daily life crumbles around her, and she finds herself slowly becoming attracted to Rem, who helps her in her predicament. On the other hand, confusion deepens with regard to her brother who carries a secret. Ritsuka is unsure what path to choose as she finds herself at the center of a love story.

Many people have fond memories of meeting a certain teenage boy in their younger years. However, their ages vary widely, and each remembers him as a teen. This story follows the intricate intertwining of their various stories and how this young man, who apparently lives outside of time, has affected each of them.

The Blue World, which symbolizes reason, is under constant attack by the creatures called the Nightfly O'Note originating from the Red World, symbolizing instinct. Akira Asagi, the 17-year-old protagonist who is a researcher at Blue World, is nominated to lead the war at Ryuukyuu LAG, a proud defense facility on the islands of Ishigaki and Iriomote. She meets the sixth combat unit "IS," which was formed four years after a fierce battle that annihilated the fifth unit. Akari must lead these six riders, also called the Scared Rider Xechs, to fight against the other world while at the same time deepening the relationship among them.

Shidou Itsuka faces greater peril than ever due to his continued involvement with Ratatoskr. He has already sealed 10 Spirits, and Isaac Westcott, leader of Deus Ex Machina Industries, has finally decided to kill Shidou and plunder the Spirits' powers for himself. To achieve his goal, Isaac declares an all-out war against Ratatoskr, forcing the organization to exhaust its resources to ensure Shidou's survival. Despite being severely outnumbered and outmatched, a glimmer of hope exists in the form of the Spirit of Time, Kurumi Tokisaki. Shidou must seal and acquire Kurumi's power to travel to the past and confront the Spirit of Origin—the catalyst that started it all.