A Janitor's Normal Day at Chaldea

In a world where all of Humanity was wiped out due to the machination of an evil god, lies the story of an ordinary person whose only qualification is that he's really good at cleaning, enabling him to become one of the Janitors for Chaldea. He only hopes that the Twins and the Servants could win the fight for Humanity. At least, there's no one who's going to bother him. Right?