[1] Two years have passed since the Unidentified Lifeform case concluded, a series of mysterious and impossible murders are taking place across the city carried out by unknown beings.

A man named Shouichi Tsugami has no memory of who he was, where he came from, or how he came upon his peculiar circumstances to transform into a powerful superhuman, Kamen Rider Agito, whenever in the presence of the beings referred to by the police as the "Unknown".

Known as the Lords, these powerful monsters perceived themselves as humanity's defenders and kill certain people in a series of murders that force the police department to make Makoto Hikawa the user of the Kamen Rider G3 powersuit which was modeled after Kamen Rider Kuuga.

Shouichi and Makoto, both initially hesitant of the other at first, are joined by Ryō Ashihara, who becomes a pre-Agito known as Kamen Rider Gills and seeks the truth behind his father's suicide.

[3] A video game based on the series, developed by KAZe and published by Bandai, was released in Japan on November 29, 2001, for the PlayStation.