"Villain of the week" (or, depending on genre, "monster of the week", "freak of the week" or "alien of the week") is an antagonist that only appears on one episode of a work of fiction. A villain of the week is commonly seen in American and Japanese 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 in 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, Smallville and Scooby Doo). 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.