An alien invader named Goa sets out to conquer Earth and first warns the Murakami family of the coming attack. To show his power, he sends father Atsushi, mother Tomoko and son Mamoru to a prehistoric jungle and destroys a giant dinosaur in front of them. The family will not surrender. Their hope lies with Magma, an armored golden giant with long hair and antennae, and his human-sized wife Mol, both made by the wizard Earth and sent to protect the world. Magma befriends Atsushi and Mamoru. The boy moves him deeply, since he longed to have a child with Mol, so Earth makes a duplicate of Mamoru called Gam. Earth also gives Mamoru a whistle that calls Gam, Mol and Magma when trouble comes. Whenever Goa releases one of his giant monsters, the three fly in to fight it in this action and science fiction mecha OVA.