Yuuta Hibiki awakens in Rikka Takarada"s bedroom and realizes two unsettling facts: he has no recollection of his past, and a strange voice from an adjacent room is calling his name. A closer look reveals a robot—introducing itself as Hyper Agent Gridman—hidden behind an old computer monitor. To Yuuta"s astonishment, Rikka neither hears Gridman nor sees the looming monsters that shroud the town in a thick mist.
Soon another colossal beast appears amid the city, causing chaos. In the midst of the turmoil, Yuuta is again pulled toward the ancient computer and fuses with Gridman. He suddenly finds himself in the thick of the fight, compelled to confront the monster. With Rikka and classmate Shou Utsumi, Yuuta establishes the "Gridman Alliance" to vanquish the monsters assaulting the city and uncover those behind their appearance.
