Tara's twentieth birthday is approaching and Willow wants them all the be at the Bronze for the party.  Tara's family gets to town and want her to come home with them.  Her father tells her that she will become a demon on her twetieth birthday.  She decided to use magic so that her friends cannot see her demon side.  The spell has further ramifications when none of the gang can see any demons.  When a group shows up to kill Buffy, Tara realizes what she's done and reverses the spell.  After the demons leave, Spike, who was helping unseen by the group, hits Tara in the face.  The pain that he feels shows the gang that she is not a demon but fully human.