Hiro is a Hunter living in Omega, a mercenary tasked with keeping the peace in a lawless, ruined wasteland where criminals roam free. Though a lone wolf, Hiro is forced to take in a strange alien girl named Empyrean upon finding her stranded after a fight. But Empyrean also carries with her the secrets of the sword Hiro now wields, called Argenti. Tasked with gathering five "Memory Keys," Empyrean strikes a deal with Hiro, forcing them to work together and confront the darkness simmering beneath their feets. If they are to restore peace to their respective worlds, Hiro must figure out the extent of his own darkness that he is willing to confront.