One day after school, first-year high school student Yomogi Asanaka encounters a starving man under a bridge. The strange drifter, introducing himself as Gauma, tells Yomogi that he is a "kaiju user," someone who deals with "kaiju"—monsters that cause harm to the city and its citizens. The next evening, Yomogi meets Gauma and his classmate Yume Minami. At the same time, a kaiju appears in a populated part of the city. Because Gauma lacks experience in controlling the kaiju, he takes a mysterious object from a glowing pouch and summons a giant robot called Dynazenon. Since the mecha requires the cooperation of four people, it pulls Gauma, Yomogi, Minami, and Koyomi Yamanaka—an unemployed man who was wandering on the street—into its cockpit. Their encounter with the kaiju marks the start of their involvement with kaiju eugenicists—kaiju users who manipulate kaiju with ill intent—and their efforts to unlock the full potential of Dynazenon.