(or "The Purest Example Ever")
Normally, when I talk about a series' storytelling engine, what I'm really doing is trying to take a look at a long-running (or occasionally short-running) series from a different perspective. Instead of just seeing the elements of the series as part of the story the writer is telling, I'm looking at them as story-generating components--the supporting cast fulfills this function, the setting adds this potential, the protagonist moves the plot this way, and so on. But it's very rare that I think that writers consciously consider their status quo as a machine that generates plots.
In the case of George Romero's seminal zombie movie series ("Night of the Living Dead", "Dawn of the Dead", "Day of the Dead", "Land of the Dead", "Diary of the Dead"), though, that's pretty much exactly what they are. Romero starts with a set of postulates that function as his "engine", and then takes other stories and runs them through the engine to see what the result will be. It's a storytelling engine that takes the world as it is, applies a major change, and observes the logical result.
The change is, of course, the dead coming back to life. Romero postulates an event (never explicated, but hinted as some sort of radiation wave released by a returning satellite) that causes every recently-deceased corpse in the world to re-animate and seek out living humans with an instinct to consume their flesh. (Their bite is invariably lethal, although Romero never makes it clear whether this is an effect of their status as zombies, or just due to the normal infections that would result from being bitten by a septic, rotting corpse.) They retain traces of their former personality, but generally have limited intelligence and diminished physical capacity (they're slower, but stronger.) Being dead, they're pretty much immune to pain, and the only way of permanently killing them is with damage to the head. But more importantly, the event affected living humans as well, even if it doesn't show. Anyone who dies in the series re-animates within minutes of their death as a zombie, unless that death is due to head trauma.
Romero's movies (and the various comic and novel spin-offs) focus on the consequences of this event for different groups. He never returns to the same set of protagonists (which allows him a lot of freedom when it comes to killing off characters), but the world is always the same. Humans find ways to survive the zombie apocalypse, some of which are co-operative (as in the small community of survivors in "Land") and some of which are competitive and counter-productive (as with the nihilistic end to "Dawn".) Different people cope with the psychological stress of the event in different ways (most of which aren't good--if Romero's movies have a common theme, it's that people tend to come unglued in crisis situations.) And the zombie horde always gets larger--in fact, with the span of time separating the movies, the size of the zombie horde provides the only definitive timeline for the series. "Diary" might look like 2005 and "Night" might look like 1968, but the two both occur early on in the zombie plague.
Romero's "zombie rules" provide a very interesting storytelling engine, precisely because they're the only real element of an engine with very loose continuity from installment to installment. This faithfulness to the rules has meant that the entire zombie sub-genre of horror has found itself defined by Romero's rules and the ground-breaking films that provided them, to the point where many zombie movies are essentially Romero movies in all but name. Some of them are loving homages, like "Shaun of the Dead", others are rip-offs, like "The Dead Next Door", and still others are deliberate reactions against or alterations of the Romero rules, like "Return of the Living Dead" or "28 Days Later" (or, for that matter, the James Gunn/Zack Snyder remake of "Dawn of the Dead".) But the Romero rules now provide a practically inescapable framework for everyone following in Romero's footsteps, a storytelling engine that has escaped its creator and run wild throughout the genre. Its simplicity is also its strength, something that is constantly proved with each new zombie movie, comic, or book that comes out.