Villain of the week
"Villain of the week" (or, depending on genre, "monster of the week") is a term that describes the nature of one-use antagonists in episodic fiction, specifically ongoing American genre-based television series. As many shows of this type air episodes weekly at a rate of one or two dozen 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. Some series alternate between using such antagonists and furthering the series' ongoing plotlines (as in The X-Files, where fandom is often divided over preference for one type of episode versus the other). Others use these one-time foes as pawns of the recurring adversaries (as in Power Rangers).
Sometimes the villain will return later in the series but now having a larger role in the story (such as the character Barry the Butcher in the Japanese manga Fullmetal Alchemist).
Use of the Format
American series
There are two different uses for this plot gimmick. Some American television series (especially animated cartoons) are designed to be purely episodic so that they can be "stripped", that is, after the initial broadcasting as a network series, usually one day a week, the film or tape is leased to independent stations to be shown five or six days a week in any order. Hence, there is no plot development, although interesting villains may appear several times (e.g., Jonny Quest's Doctor Zin made four appearances in the original series).
Another reason for using this format is that it is convenient for writers to supply a continuous and varied amount of challenges for the protagonists to overcome. One perceived "flaw" to continuity-based series is that, if the show is based upon a single dominating plot device (ie: defeating a single reappearing adversary), then should that plot device ever be resolved, the series would supposedly "end". Conversely, if the plot device is not resolved eventually, the premise of the show may become stale. Therefore, a lack of major continuity is often thought to be a convenient solution.
However, in recent decades, many American animated series have shifted away from this style. A prominent example is the DC Animated Universe, which is covered from Batman: The Animated Series to the current Justice League Unlimited. While the former series was mostly episodic, with only moderate continuity between episodes, Unlimited is very continuity-heavy--even making continual references to past series. Other American series (both live-action and animated) have also adopted more plot-based continuity. 24, Jackie Chan Adventures, Lost, Gargoyles, and The Sopranos are shows that placed varying levels of importance of continuity.
Import Editing Controversy
In Japan, continuity in animated television series is used more frequently than in the United States. A significant number of series are designed to have an overall plot, even if it's just an initial episode and one or two concluding episodes. Sometimes, most of the intervening episodes have no plot or character advancement (e.g., the early Planet Boy Popi, called Prince Planet in America), but other shows like Mobile Suit Gundam have their plot based on a journey and use character development throughout the entire series. Sometimes, American distributors use exisiting footage from Japanese shows and use it for the "villain of the week" format. An example of this is the live action series Kyoryuu Sentai ZyuRanger. Footage from this, mostly involving the monsters, was edited together with American-made footage in the production of the first season of Mighty Morphin Power Rangers. However, as Japanese animation has become more poluar with the American public, anime has become more popular with American markets, most imported Japanese shows are shown with minor edits or no editing at all.
Other Examples
Series that make frequent (or, in some cases, near-constant) use of the villain/monster-of-the-week formula include: Buffy the Vampire Slayer, Charmed, Hercules: The Legendary Journeys, Jackie Chan Adventures, Kirby: Right Back At Ya!, Sonic X, Power Rangers, Sailor Moon, Bomberman Jetters, Smallville, Tokyo Mew Mew, MegaMan NT Warrior, Ultraman, Teen Titans, Xena: Warrior Princess, and The X-Files.