Sore ga Kanojo no Seigi nara

After his happy-go-lucky lovestruck parents suddenly decide to go on an amorous traveling adventure, Yuki is shuffled off to live with one of his dad's old friends. He knows very little about his new living circumstances, save that his dad's friend is an author with three children.
Something seems a little off about the girl who's sent to meet him, the fidgety and overly anxious Aya, but he doesn't fully understand what's going on until he arrives at the house. Yuki, who has had very little to do with women thus far in his life, is now set to live in a house with four of them. The head of the house is actually an author of erotic novels, and she habitually forces her three daughters into embarrassing sexual situations. How will Yuki cope with his new life?