All episodes

2x07&08 Lie to Me/The Dark Age

2x05&06 Reptile Boy/Halloween

2x03&04 School Hard/Inca Mummy Girl

2x01&02 When She Was Bad/Some Assembly Required

1x11&12 Out of Mind, Out of Sight/Prophecy Girl

1x09&10 The Puppet/Nightmares

1x08 I Robot, You Jane

S1E06&07 The Pack/Angel

1x05 Don't Kill a Boy on the First Date

1x04 Teacher's Pet

1x03 The Witch

1x01&02 Welcome to Hellmouth/The Harvest