In the year 2005, a race of alien monsters known as the Zonders emerge from beneath the earth and begin assaulting Tokyo. The only line of defense against these creatures is the secret organization called the Gutsy Geoid Guard, abbreviated as 3G, along with their ultimate weapon: the massive giant robot GaoGaiGar. The pilot of GaoGaiGar, Guy Shishio, was a former astronaut who nearly died two years earlier when the Zonders first impacted Earth. His life was saved when a mysterious robotic lion named Galeon pulled him from the burning shuttle and brought him to the planet. Guy's father, Leo, then employed Galeon's technology to reconstruct his severely injured son as a cyborg, hoping he would be able to stop the aliens when they finally appeared. Now, with Galeon serving as its core, GaoGaiGar fights to defend Earth. He is supported by a team of transforming robots and a young boy named Mamoru, who possesses the ability to purify the Zonders' cores and appears to have a link to the enigmatic Galeon.