An artifact has been forged which enables time-travelling spells while it remains in Sunnydale. A group of vampire-assassins are travelling into the past in an attempt to kill previous Slayers, and disrupt the Slayer lineage. They are led by the spellcaster Lucien, whose aim is to ensure that Buffy does not interfere with the ascension of the Master. When they discover that killing Buffy in the past merely changes the way in which the Master is killed, they become frustrated and decide to go further back.