Tokisaka-san wa Boku to Chikyuu ni Kibishisugiru.

Utsugi Sou, a high schooler that is very serious about taking out garbage on a daily basis. He lives alone and has a close girl friend. Lately, his garbage has been messed with and redivided by a stranger unknown. And, one day, a transfer student, Tokisaka Koyomi arrives and seems to know him; however, he has no clue. Later, he finds out that the transfer student is from the future. The story begins from here…

In fifteenth century Rome, witches have declared war against humanity. A young man named Nikola, whose parents were cruelly murdered by a witch before his eyes, discovers that he is now the key to winning the war. Using sorcery skills acquired through years of careful study, Nikola must revive twelve witches known as the "First Witches" who will become his allies and weapons to defeat the enemy. But there is one small hitch: in order to revive the witches from their eternal slumber, Nikola must form a marriage contract with each one. Just how many witches can Nikola marry in order to save the world?