For years, the Japanese government has kept a secret: mythical creatures such as centaurs, harpies, and lamias are real.^[8] Three years before the start of the story, the government revealed the existence of these creatures and passed a legal bill, 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 restriction being that liminals and humans are forbidden from harming each other).

Kimihito Kurusu did not volunteer for the exchange program, but when Ms. Smith delivered the very scared and embarrassed Miia to his door by mistake,^[8] he did not have the heart to send her away and they started living together. As the story continues, Kimihito meets and gives shelter to other female liminals, each of a different species. Some arrive more or less by accident, some are forced upon him by Ms. Smith, or force themselves in, and it does not take long for him to find himself in a hectic environment where he struggles to live in harmony with his new housemates while dealing with both their constant advances and the dramas of helping them get along in the human world. The situation takes on a new twist after he is told that because of expected changes in the law dealing with human-liminal relationships, it is hoped that he will marry one of the girls.