(from MangaUpdates):

The secret society "Big Fire", scheming to conquer Earth, is furthering Project GR. The special investigation organization of the United Nations dispatches an agent in Country T to interfere with the plan. Daisaku Kusama, a Japanese tourist, is mistaken for the agent and is abducted by "BF". While he is held hostage, a freak accident occurs leaving Daisaku the only person on Earth able to control the worlds most powerful robot, Giant Robo, and is enrolled by the UN to destroy Big Fire's ambitions with their own creation.