For years, the Japanese government had kept a secret: mythical creatures such as centaurs, mermaids, harpies, and lamiais are real. Three years before the start of the story, the government revealed the existence of these creatures and passed the "Interspecies Cultural Exchange Act". Since then, these creatures, known as "liminals", have become a part of human society, living with ordinary families like foreign exchange students and au-pair visitors, but with other duties and restrictions (the primary restrictions being that liminals and humans are forbidden from harming each other or procreating).

In Asaka, Saitama, you’re a normal student who initially has no connection to the exchange program. However, when coordinator Kuroko Smith delivers the very scared and embarrassed Miia to your door by mistake, you cannot bring yourself to send her away and allows her to live at your home, taking advantage of your parents' extended absence. As the story continues, you met and gives shelter to other female liminals, each of a different species. Some arrive more or less by accident, while others are forced upon you by Smith or simply insinuate themselves into the house. Before long, you find yourself in a hectic environment, struggling to live in harmony with your new housemates while dealing with both their constant affectionate/romantic advances and the dramas of helping them get along in the human world.