For the last three years, Clay has undergone rigorous training in order to find her father, who was last seen exploring a dungeon. While dungeon exploring is difficult for any regular adventurer, Clay knows her father, a thief who doesn't purely rely on raw power or magic, managed to progress farther than anyone else. Clay sets off after him, following in his footsteps until she encounters a furnished room behind a dungeon wall. Clay is shocked to find the room is filled with monsters that can talk and a girl who claims to be the dungeons administrator. However, the room is meant to be kept hidden from explorers. Now that she knows the secret behind the dungeon, Clay is hired to be part of their staff. While she never expected to find a job in her adventure, doing so may just help Clay unravel the mystery behind her father's disappearance.

The year is 20XX. Mysterious huge robots called "Giant Robo (GR)" started to appear all over the world and destroy cities. The earth was covered with fear. And their next target is Japan! The hero of this animation Daisaku Kusama, who works in a diving shop, encounters a mysterious girl, simply called V who leads him to make a contract with an enormous robot "Giant Robo 1 (GR1)" at the ancient ruins (Minami Yonaguni Island) in Okinawa. He made a contract with UNISOM (good guys) and was approved to be a commander of GR1. Thus his battle has begun against GRO (bad guys) and creates a lot of dramas. Before long, Giant Robo of the 21st century will ask the world the true meaning of its power. (Source: AniDB)