A wormhole in space forms just outside the Earth's atmosphere and monstrous creatures descend upon the world's cities. A top-secret global defense organization code-named GUARD is established to counter the attacks. Young GAM TAKAYAMA discovers the power to transform himself into the gigantic superhero ULTRAMAN GAIA, and joins X.I.G., GUARD's front line defense team. A second Ultraman appears. But is this new colossus friend or foe?