For years, the government has kept a secret: mythical creatures are real.
Three years before the start of this story, the government revealed the existence of these creatures and passed a legal bill, the "Interspecies Cultural Exchange Program". 
Since then, these creatures 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 they and humans are forbidden from harming each other or fornicate without being married).

The government is still actively seeking out those that break the laws of the program. But sometimes a bit of mischief happens right underneath their noses...