Last night’s How I Met Your Mother was a “mythology episode,” a half-hour that pushed forward the big mysteries of the show: how did Ted meet the mother of his children, how does the yellow umbrella figure into it, did we actually see her onscreen. It’s the kind of thing that we’re all familiar with from The X-Files and Heroes and Lost: these shows have stand-alone stories and/or current storylines, but they’ll also devote some time to building up the overarching mysteries that may not get solved until the show ends. Some of this comes from comic books, which require villain-of-the-month stories to be balanced with character issues that will never be truly resolved: a Batman story is about him fighting a bad guy, the Batman mythology is all the stuff about his parents and his childhood and various lovers/sidekicks/wards.
Obviously the idea that a show has to have its own continuing, internally consistent mythology is a fairly new development. Older shows were required to have every episode self-contained, so there was no question of an overall mythology — or, more accurately, it was up to the fans to create the mythology in their own mind. (There is a Star Trek mythology, but it’s entirely created by fans who extrapolate from individual episodes they’ve seen and apply these mythological elements to the characters and the whole series. But the writers never felt a need to keep these things consistent from episode to episode.) Even more recent shows that did have continuing story arcs did not necessarily find it important to create an internally consistent set of rules for their universe. Buffy the Vampire Slayer was the sort of show that could have had a mythology, and fans often talk about it as if it did, but it really didn’t; rules were constantly brought up and then discarded, and while there were references to previous episodes, every story arc was really more its own thing that the writers made up as they went along. (Introducing an imaginary sister for Buffy was not part of a long elaborate mystery as it would be on Lost. It was just a story idea the showrunner had kicking around and finally got around to using in the fifth season, but it was not actually connected to any previous events.)