In the year 2005, a mysterious alien force known as the Zonders emerges from underground, launching a series of attacks on the city of Tokyo. The only defense against these creatures is the secret agency known as the Gutsy Geoid Guard (or 3G) and their ultimate weapon, the powerful giant robot GaoGaiGar. GaoGaiGar's pilot, Guy Shishio, is a former astronaut who was nearly killed two years prior when the Zonders first crashed to Earth. With the aid of the mysterious robot lion Galeon, which saved Guy's life by pulling him from the burning shuttle, his father Leo rebuilt Guy as a cyborg, utilizing Galeon's technology in the hopes that he could stop the aliens when they appear. GaoGaiGar fights to protect Earth, aided by a team of transforming robots and a young boy named Mamoru, who possesses the power to purify the Zonders' cores and seems connected to the enigmatic Galeon.