
Shouichi Tsugami has no memory of his past. He doesn't know his identity, his origins, or how he ended up in his strange situation. For no apparent reason, Tsugami transforms into the formidable superhuman, Agito, whenever he encounters the creatures the police call the "Unknown"—a terrifying species of monsters responsible for a series of murders in Tokyo, where they select specific individuals as their victims. In response, the police deploy their latest weapon: the G3 powersuit, originally designed to combat the "Unidentified Lifeforms" that oppose No.4 (Kamen Rider Kuuga). G3 and Agito are uncertain whether they should team up to fight the Unknowns, their shared enemy, or continue their rivalry, keeping their intertwined secrets apart. As events unfold, even greater mysteries emerge with the arrival of Kamen Rider Gills, who seeks to discover the truth behind his father's suicide. These mysteries converge, and the true identity of Agito will ultimately decide the future of humanity.