One Summer Ago

Kize is just a normal student living in Hokkaido, until a young female, Riyoku, a transfer student from Shirakawago, starts at his school. That was one year ago.

Now, Kize's got girls begging him for a date, and boys who constantly try to hook up their sisters with him. Little do they know, Kize's has an arranged marriage with Riyoku, and he's going to marry her on the day she turns eighteen, which is in a month's time.

The night before the wedding, Riyoku goes missing, causing chaos in the household. It's up to Kize to save his fiancée and kill certain monsters, called Entities...