How does Buffy go on with her memory in shambles - without her purpose?

It keeps her up at night, the powerlessness and the jealousy... but she's soon thrust into a monstrous battle nonetheless as a dark creature with an appetite for Slayers still thinks Buffy is the real thing! Will Willow be able to help in time, or will Buffy find herself alone and in the clutches of a terrifying evil?