BUFFY THE VAMPIRE SLAYER S3e16 Dopplegangland

The one where Enya & Willow cast a spell that backfires resulting in alternate reality-evil-vampire-Willow being displaced to the wrong universe. Best parts: They swap clothes, leaving evil-Willow in the pink fluffy outfit and good Willow in black leather dom-wear so she can infiltrate the bronze; evil Willow beats up slacker football guy and later he treats Willow way better, Willow has the best lines, especially when she was describing vamp- willow, "skanky and kind of gay".