An alien weapon arrives on Earth with no memory of what it is, and spends thirteen episodes choosing humanity over its own species one quiet decision at a time. Mars doesn't explode into epic battles to earn this choice. It whispers it through a man learning to want something other than his programming, through fragments of a woman's face that won't leave him alone, through the weight of knowing that loving Earth means dying with it. The real threat isn't invasion. It's the possibility that Mars will remember who he was supposed to be.

A young man with no memories is found stranded on a newly formed volcanic island by Iwakura, a journalist reporting there. A mysterious figure named Rush tracks him down and tells him who he is: his name is Mars, and like Rush he comes from beyond Earth. Mars was sent to command the immensely powerful robot Gaia and destroy the planet once humans try to push their influence out into space. Because of his amnesia, Rush grants him ten days to study Earth's history and decide what to do. Haunted by images of a woman urging him not to destroy the world, Mars chooses to fight for Earth, even against his own kind. The stakes are grave: Gaia is built to self-destruct if Mars dies, and that blast would take the whole planet with it.