When Kanami sees a black-haired, black-eyed girl named Maria at a slave market, she reminds him so much of his little sister he had to get her out of there. But his motives aren't totally altruistic since he wants to use her as a dungeon diving companion. Maria stubbornly refuses to see their relationship as anything but master and slave, but over time she gradually warms to Kanami. Will she use her unique skills to help him defeat the Dungeon?