In 2005, a race of alien monsters known as Zonders emerges from underground to launch a series of attacks on Tokyo. The sole defense against these creatures is the secret agency called the Gutsy Geoid Guard (or 3G) and their ultimate weapon, the formidable 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. Guy survived because a mysterious robot lion named Galeon rescued him from his burning shuttle and brought him to Earth. Guy's father, Leo, then used Galeon's technology to reconstruct his shattered son as a cyborg, hoping he could stop the aliens when they appeared. Now, with Galeon as its core, GaoGaiGar fights to protect Earth. He is assisted by a team of transforming robots and by a young boy named Mamoru, who possesses the power to purify the Zonders' cores and appears to be connected to the mysterious Galeon.