Blood Ties

In which Dawn suffers a crisis of self. Just the normal stuff that any teenage girl who was created out of glowing green energy by a group of monks trying to hide a key from a hellgod goes through. By the way, Ben is Glory. Glory is Ben.

Official back-of-box synopsis:

As Buffy celebrates her 20th birthday, her sister Dawn finds out she is 'the Key'. Confused and upset, Dawn runs away and Buffy must find her before Glory does.

From the old WB Buffy website:

DAWN IS UNVEILED AS THE KEY - As Buffy and friends celebrate her 20th birthday, Dawn is traumatized to learn she is the Key and runs away but Buffy and the Scooby gang must find her before Glory does.

Cross-reference:

episodes featuring Buffy's birthday:

• Surprise
  Buffy is 17, and takes her relationship with Angel to a new level.
• Helpless
  Buffy faces her ultimate trial, and her ultimate betrayal, on her 18th birthday.
• A New Man
  Giles is transformed by Ethan as Buffy celebrates her 19th.
• Older And Far Away
  Buffy is 21, and throws the weirdest, possibly the longest, birthday party ever.
• this episode
  Dawn goes missing on Buffy's 20th.

episodes showing Buffy as a child:

• Killed By Death
  Buffy plays superhero, but is unable to save her cousin
• The Weight Of The World
  Part of Buffy's dream takes her back to her childhood
• this episode
  Dawn relives "memories" of her childhood with Buffy

Cast:

• Sarah Michelle Gellar as Buffy
• Alyson Hannigan as Willow
• Nicholas Brendon as Xander
  

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• Anthony Stewart Head as Giles
• Kristine Sutherland as Joyce
• James Marsters as Spike
• Emma Caulfield as Anya
• Amber Benson as Tara
• Michelle Trachtenberg as Dawn
• Clare Kramer as Glory
• Troy T Blendell as Jinx
• Justin Gorence as Orlando
• Carl J Johnson as Crazy Guy #3
• Michael Emanuel as Burly Guard
• Joe Ochman as Janitor
• Candice Nicole as Buffy as a child
• Elyssa D Vito as Dawn as a child