One reason “The Simpsons” has run long enough to sustain an uninterrupted 12-day marathon is that nothing much changes around Springfield. The kids don’t age, and what happened, oh, 17 years ago doesn’t have any bearing on this week’s events.

But the days of the footloose, utterly self-contained episode are slowly fading. In fact, the heavy serialization of TV — and the pleasures of a novelistic relationship with shows — has bled into the sitcom and the dramatic procedural formats, which once derived much of their power from offering audiences a similar experience time and again. So much for “Bonanza’s” Cartwright boys bringing order to the Ponderosa in an hour, and starting all over again next week.

Place some blame for this on “The Blacklist,” the NBC drama that has successfully mixed catching a bad guy every week (chosen from a convenient inventory, no less) with a deeper, ongoing mystery about the relationship between the central characters. So come for the ruthless dispensation of justice, stay for the “So is Red her father or not?” clues and speculation.

Tellingly, this strategy has found its way into many of this fall’s hours, which otherwise might look like old-fashioned procedurals. The first episodes of ABC’s “Forever” hint at a darker mythology in the form of a faceless threat, while even CBS — the network that’s built on an alphabet soup of crime shows and spinoffs — felt compelled to incorporate a mystery into its new drama “Madam Secretary.” That’s something of a breakthrough for a network that’s launching additional “NCIS” and “CSI” franchises as if they were Starbucks.

Serialization also has become a component of more sitcoms, such as CBS’ “Mom,” which has progressed through an unplanned pregnancy and the characters’ relationship and financial woes.

Admittedly, the trend isn’t entirely new, but for networks that once counted on an audience coming back for “CSI” or “Law & Order,” it’s noteworthy nevertheless. Because once shows begin planting these seeds of mythology — say, the looming threat of an alien invasion, in the case of “The X-Files” — it can be difficult to ignore the existential challenge and go back to chasing that week’s monster in the woods.

Indeed, networks used to relish a certain disposable quality to their wares, maintaining that viewers didn’t watch every episode of even favorite shows, which meant broadcasters could rerun “Roseanne” and expect a subset of its fans to show up. Hence NBC’s much-lampooned campaign that tried to sell repeats with the come-on that if you haven’t seen them, then “It’s New to You.”

In some respects, all this is potentially good news for networks, which can market the greater passion, engagement and sense of urgency that comes with serialized storytelling. And while not everything can be “Game of Thrones,” viewers ostensibly benefit from the richness and ambition associated with seeing characters develop and stories arc, even when that’s measured in inches.

At the same time, though, such an evolution weakens the stability that came from a network creating a hit crime, legal or medical franchise, then sitting back and having viewers dutifully tune in just to see Kojak, Perry Mason or Marcus Welby do their thing.

Broadcast networks already have made numerous adjustments to their traditional model — including a spate of limited series — and this new-look procedural, spiced with a sprinkling of serialized garnish, is just one more acknowledgement that a mimeographed approach isn’t as exciting or reliable as it once was.

As for what that means for the procedural’s future, with this season’s new shows rolling out in earnest, well, that’s to be continued. And in the meantime, don’t have a cow.