After waking up in Rikka Takarada's room with no memories, Yuuta Hibiki hears a mysterious voice calling him from a nearby room. Investigating, he discovers Hyper Agent Gridman, a robot residing in an old computer. Surprisingly, only Yuuta can communicate with Gridman, and he alone can see the ominous monsters shrouded in fog outside. When a giant monster attacks the city, Yuuta merges with Gridman and joins the battle. With the help of Rikka and Shou Utsumi, they form the 'Gridman Alliance' to combat the city's threats and uncover the source behind the monster appearances.