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Warning, spoilers within.

Season 1
Welcome to the Hellmouth/Harvest
The Witch
Teacher's Pet
Never Kill a Boy on the First Date
The Pack
Angel
I robot, you Jane
Puppet Show
Nightmares
Invisible Girl
Prophecy Girl
 

Season 2
When she was bad
Some assembly required
School Hard
Inca Mummy Girl
Reptile Boy
Halloween
Lie to me
The Dark Age
What's my line? Part 1
What's my line? Part 2
Ted
Bad Eggs
Surprise
Innocence
Phases
Bewitched, Bothered, & Bewildered
Passion
Killed by Death
I only have eyes for you
Go Fish
Becoming Part 1
Becoming Part 2
 

Season 3
Anne
Dead Man's Party
Faith, Hope, & Trick
All men are beasts
Homecoming
Band Candy
Revelations
Lover's Walk
The Wish
Amends
Gingerbread
Helpless
The Zeppo
Bad Girls
Consequences
Dopplegangland
Enemies
Earshot
Choices
The Prom
Graduation Part 1
Graduation Part2