One day after school, first-year high school student Yomogi Asanaka came across a starving man under a bridge. The man introduced himself as Gauma, and claimed to be a "kaiju user," someone who deals with monsters that bring harm to the city and its citizens.
The next day, Yomogi ran into Gauma and his classmate Yume Minami, and at the same time, a kaiju appeared in a populated area of the city. Gauma tried to control the creature, but was unable to do so. In a last-ditch effort, he opened a glowing pouch and summoned a giant robot known as Dynazenon.
The robot required four people to work together in its cockpit, so it dragged Gauma, Yomogi, Minami, and Koyomi Yamanaka, an unemployed man wandering the streets, into the cockpit. This marked the beginning of their entanglement with a group of kaiju eugenicists, people who manipulate kaiju with malicious intent, and their attempts to bring out the full potential of Dynazenon.