Peaceful days in Tsutsujidai City were disrupted by the attacks from Kaiju. Yuta Hibiki, an amnesiac first-year high school student, met the Hyper Agent Gridman in an old computer and took up the mission to protect the city with his classmates by merging with Gridman to fight against Kaiju. However, after the Kaiju attack, people's memories were reset and those who died were completely forgotten as if they never existed. Could Yuta and his friends stop the attacks from Kaiju and uncover the truth behind the city? Where did the Kaiju come from?