Planet Earth is under attack by an alien invader known as Goa. In an effort to assert his dominance over the planet, Goa abducts the Murakami family, consisting of father Atsushi, mother Tomoko, and son Mamoru, and transports them to a prehistoric jungle where he showcases his power by vanquishing a colossal dinosaur. Despite their refusal to surrender, the family is given a glimmer of hope when Magma, a golden armored giant, and his human-sized companion, Mol, arrive to help. These two warriors were designed by a wizard on Earth to thwart Goa's plans for domination. As Atsushi and Mamoru bond with the heroes, Magma becomes particularly touched by Mamoru's desire to have a child with Mol. In response, Earth constructs "Gam," a clone of Mamoru, and presents Mamoru with a whistle to summon the trio of Magma, Mol, and Gam when needed. With Goa's army of daikaiju monsters causing destruction, Magma, Mol, and Gam are always ready to lend a hand.