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II. Story
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(Taken directly from the instruction manual.)

When Buffy Summers discovers that an enemy from the past, Ethan Rayne, is
at the center of a great struggle with The First, literally the first
incarnation of evil the world has ever known, she and the gang must face
an undead army of vampires, zombies, and demons to keep these nefarious
villains from casting the world into permanent darkness.

It would help you a great deal if you were a fan of the series or at least
watched it.  That will provide all of the back story you will need.