Plot generator
A plot generator is a fictional plot device which permits the generation of plots for an extended serial without requiring a great deal of logical connection between the episodes. A plot generator is found in any set-up which will produce an ongoing series of new cases, issues, etc., as in detective or police serials (incoming criminal cases); medical drama (incoming medical cases); and so on. Even more broadly, "what if" stories can permit writers to introduce any idea they want, as in The Twilight Zone; for obvious reasons, this type of plot generator is found generally in the science fiction and fantasy genres, especially in episodic series which have a villain of the week format.
Examples of plot generators
The idea of plot generators is explored by Nick Lowe in his article "The Well Tempered Plot Device". Examples he gives include "red kryptonite", which has an unpredictable effect on Superman for 48 hours (conveniently the time period covered by most episodes).
The TARDIS in Doctor Who is the epitome of a plot generator, in that it can take the Doctor anywhere in space and time, where he can encounter anything the writers want him to encounter, from aliens in outer space to famous historical figures.
The Stargate from the movie Stargate, and the television series based on it, Stargate SG-1, is an effective plot generator. By activating and stepping through the gate the characters are taken to any one of a very large unspecified number of alien worlds.
The Holodeck, introduced in the episode The Practical Joker of Star Trek: The Animated Series but more prominently seen on Star Trek: The Next Generation, could also be viewed as a plot generator, as its open ended nature allowed the show's writers to construct a number of stories throughout the series.
Enid Blyton's three books about The Faraway Tree feature a magical tree with a rotating system of worlds which can be reached by climbing to the top of the tree.
Law & Order, and to a lesser extent its spinoffs, has a structure that serves as a plot generator: few episodes are about the characters' interactions with each other; nearly all are, in a villain of the week format, about the detectives solving, and the lawyers prosecuting, the mystery at hand.
Quantum Leap features a time traveling protagonist who is lost in time. Due to a botched time travel experiment, the character is perpetually time traveling to different era/place once the storyline is conveniently resolved at the end of each episode.
The television show Sliders featured a device that could send the main characters to any one of infinite alternate dimensions, each episode featured the group appearing in at least one new dimension, which usually had no particular connection to previous episodes.
The Sunnydale Hellmouth in Buffy the Vampire Slayer and the Cardiff spacetime rift in Torchwood act as plot generators, explaining the presence of incredibly evil humans, demons, erratic behaviour, magical anomalies etc. or the presence of alien technology, extraterrestrial life, psychic powers and time abnormalities respectively.