A man named Mars, who cannot remember his past, is discovered on Heisei, an island with a growing volcano. What nobody realizes is that Mars is an alien sent to gather information on whether Earth should be destroyed. Despite troubling hallucinations of a woman pleading with him to spare Earth, Mars teams up with a massive robot named Gaia to defend the planet rather than destroy it. This decision means fighting against other extraterrestrial beings. The threat is heightened as Gaia will explode, destroying the world, if Mars fails.