Based on the adult doujin by Yasuyuki Watarai.

Ema is a good girl living alone without a parent. Only his grandfather left him to replace him as his father Ema. They live together in a spacious house. With a lecherous upbringing of his grandfather who is regarded as Father, Ema increasingly grows into a nasty person over the upbringing and affection of a fanatic from his grandfather.

(Source: Nekopoi and Has Been Translated)