"Villain of the week" (or, depending on genre, "monster of the week", "freak of the week" or "alien of the week") is a term that describes the nature of one-use antagonists in episodic fiction, especially ongoing American genre-based television series. As many shows of this type air episodes weekly at a rate of ten to twenty new episodes per year, there is often a new antagonist to forward the plot of each week's episode. The main characters usually confront and vanquish these characters, often leaving them never to be seen again (as in Charmed, Scooby Doo and Sailor Moon). Some series alternate between using such antagonists and furthering the series' ongoing plotlines (as in Buffy the Vampire Slayer, Fringe and The X-Files, where fandom is often divided over preference for one type of episode versus the other), while others use these one-time foes as pawns of the recurring adversaries (as in Kamen Rider, Super Sentai and their American equivalent, Power Rangers). On other occasions, these villains return reformed, becoming invaluable allies or gaining a larger role in the story.