For as long as humanity has existed, there have been "Others" among us; Witches, Vampires and Shape-Shifters who are soldiers in the eternal war between Light and Dark.

Light Others protect mankind from Dark Others, who plague and torture humans.
TAKEN FROM THE OFFICIAL WEBSITE:
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.

Ancient prophecy foretells that one day the Great One will arrive who can end the threat of an apocalyptic battle between Light and Dark Others. That day has come, and the Great One, once he or she is identified, must choose whether to destroy the light within or battle the surrounding darkness. This choice will reveal mankind's destiny.

Exciting News-There is to be a third movie in 2009