Gingerbread

In which Joyce finds her purpose; it was hidden behind that bonfire. No, not that one; the one on which Buffy and Willow about to be immolated. It's good that she's got a life outside the home.

Official back-of-box synopsis:

When two young children are found dead, the whole town rallies together and blames the death on witchcraft. When witch paraphernalia is found in Willow's locker, no-one believes it was for a special birthday present she was preparing for Buffy.

From the old WB Buffy website:

THE SUNNYDALE WITCH HUNT - Picking the worst possible night for a surprise "mother-daughter bonding" visit with Buffy, Joyce is haunted by the discovery of two murdered children and feels compelled to take drastic action. When Giles suggests that the killings appear to be an occult sacrifice, Joyce rallies the entire adult population of Sunnydale behind a Salem-like witch hunt, leading the murderous mob directly to Willow and Buffy.

Cross-reference:

episodes with scary Mothers:

• The Witch
  Amy's mom takes over her body, not just her life
• Some Assembly Required
  Mrs Epps, there's surgery going on in your basement!
• Conversations With Dead People
  OK so it wasn't Joyce, but it was very scary
• Lies My Parents Told Me
  Spike's mother, enough said.
• this episode
  Nice witch-hunt, mom!

Cast:

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

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• Anthony Stewart Head as Giles
• Charisma Carpenter as Cordelia
• David Boreanaz as Angel
• Kristine Sutherland as Joyce
• Elizabeth Anne Allen as Amy
• Armin Shimerman as Principal Snyder
• Seth Green as Oz
• Harry Groener as The Mayor
• Jordan Baker as Sheila Rosenberg
• Lindsay Taylor as Little Girl
• Shawn Pyfrom as Little Boy
• Blake Swendson as Michael
• Grant Garrison as Roy
• Roger Morrissey as Demon
• Daniel Tamm as MOOster