The protagonist and his four sisters got separated because their parents divorced. During the summer vacation he receives a request from his mother to take care of his sisters while she's away. Not having seen his sisters in years, he willingly accepts.

But once he's there he eats a poisonous mushroom given to him by his father intended to celebrate the reunion. The poison affects the nervous system and the symptoms include an increase in sexual desire and increased sperm production. Moreover, if you let it build up too much your own precious mushroom will blow off.