A magical space called "Contraindication Zone" that suddenly appeared all over the world. The monster "Eater" that appears there and eats the human spirit Mankind was powerless to Eater, who was ineffective against all physical and nuclear attacks ... However, humankind is not powerless forever. Attack technique using bioenergy called "Mana" --- A warrior who can manipulate magic and defeat the only eater People called them "witch" witches --- A witch school that trains witches and their partner "Familiar". The main character, Toshiya Iizuka, failed the entrance examination, but became a student in the form of an invitation admission after finding an unequaled and big cock. In this world, men who cannot use magic are required to supply "mana", which is the source of magic for witches. Men are converted to "manna = sperm" in their bodies, so instead of being unable to use magic By giving sperm to the witch, that is, sexual activity, it is possible to replenish manna semi-permanently. In this way, Toshiya's life at Witch School began.