The Alternative Factor (S01E27)

S01E01 The Man Trap S01E02 Charlie X S01E03 Where No Man Has Gone Before S01E04 The Naked Time S01E05 The Enemy Within S01E06 Mudd's Women S01E07 What Are Little Girls Made Of? S01E08 Miri S01E09 Dagger of the Mind S01E10 The Corbomite Maneuver S01E11 The Menagerie (1) S01E12 The Menagerie (2) S01E13 The Conscience of the King S01E14 Balance of Terror S01E15 Shore Leave S01E16 The Galileo Seven S01E17 The Squire of Gothos S01E18 Arena S01E19 Tomorrow Is Yesterday S01E20 Court Martial S01E21 The Return of the Archons S01E22 Space Seed S01E23 A Taste of Armageddon S01E24 This Side of Paradise S01E25 The Devil in the Dark S01E26 Errand of Mercy S01E27 The Alternative Factor S01E28 The City on the Edge of Forever S01E29 Operation: Annihilate!