Yomogi Asanaka, a first-year high school student, comes encounters a hungry guy under a bridge one day after school. The mysterious traveler introduces himself as Gauma and tells Yomogi that he is a "kaiju user," a person who deals with "kaiju"—monsters that do harm to the city and its residents. Yomogi bumps into Gauma and his classmate Yume Minami the following evening. A kaiju emerges in a populous region of the city at the same time. Due to Gauma's lack of familiarity with kaiju, he summons a massive robot known as Dynazenon by pulling a mystery object from a bright pouch. The mecha drags Gauma, Yomogi, Minami, and Koyomi Yamanaka—an unemployed guy who was wandering the streets—into its cockpit, requiring the collaboration of four individuals.