The beings of earth have been cursed and to save them someone implemented a system made from human games and books to save everyone from dying by their own power.

Right as the system took effect, Jude was summoned by an alien mage into the middle of a super easy battle. Whatever force that kept humans from being summoned as monster defenders has been removed.

After Jude quickly killed the weak little lizards attacking everyone, the mage attempted to banish him back to his home plane and then the mage exploded, leaving Jude stranded on an alien world.