Every generation, the Power of Three defends the world of witches and mortals from the demons, warlocks, and assorted other monsters that seek to corrupt and destroy them. The whitelighters tell them how to do that, and they save the innocents: mortals who don't know about the Masquerade, as well as other magical beings who are not aligned with Evil. Various other levels of celestial bureaucracy interfere: above the whitelighters are elders, and besides those are angels of destiny, cleaners, etc.