In 1998, Espenson joined Mutant Enemy Productions as executive story editor for the third season of "Buffy the Vampire Slayer". Over the rest of the run of the series, Espenson wrote or co-wrote twenty-three episodes, starting with "Band Candy" and ending with "Buffy"s penultimate episode, "End of Days". After her role as an executive story editor, she was promoted to co-producer in season four. In the fifth season she was promoted again to producer. She took up the role of supervising producer in the sixth season and was promoted once more to co-executive producer in the final season.