The opening shows our boy having haremlike close encounters with a half dozen busty monster girls
Miia is here on an interspecies exchange program. Once the government stopped keeping the existence of demi-humans secret, the program was set up to help them integrate.
Our boy is Kimihito Kurusu, and he seems to have accidentally become a host family while his parents are away. Oops. Some kind of paperwork screwup, no doubt.
Smith, the interspecies coordinator drops by. She's noticed that Miia is desperately in love with him.