Light Others protect mankind from Dark Others, who plague and torture humans.
Over 1000-years ago a truce was struck between Gesser, Lord of the Light, and Zavulon, General of Darkness. They agreed that no one could be forced to good or evil, people must choose freely for themselves. To uphold this truce, each side established underground forces; the soldiers of the Light would be called Night Watch, making sure Dark Others obeyed the truce. And the soldiers of Darkness would be called Day Watch, to do the same.
As i said a simaler plot to constantine, where the evil was trying to change a persons life so as not to go to heaven