Koi Suru Pecora wa Machigaenai

There's only one day before Halloween. Sister Eri (name changeable), who lives in a peaceful town, receives a warning from a strange fortune teller. "If things stay like this, something bad is going to happen to you on Halloween, dear lady." The only way to avoid it is to deal with the "little bad things" surrounding her. Will she be able to overcome the ordeal safely?