Ask any casual fan of Buffy the Vampire Slayer to name their favorite episode and you’d get a veritable cornucopia of options. Perhaps they’d pick the near-wordless grim fairytale of “Hush,” the wrenching grief of “The Body,” the Broadway-quality musical that is “Once More With Feeling,” or the twisty dream logic of “Restless.” Twenty years since the show premiered on The WB, individual episodes stand out and are treated with the reverence afforded cult films, their relative virtues endlessly debated online, their details picked over by scholars.

But the same casual fan could also tell you their favorite season of the show—be it the epic romantic drama of its second (with Angel’s turn to evil), the graduation angst of the third (with the vampire slayer rival Faith), or the celestial battles of the fifth (pitting Buffy against the demon goddess Glory). It might seem obvious now that a popular TV show would center its viewing experience on both great individual episodes and long, serial arcs that played out over an entire season, but when Buffy did it in the late ’90s, it was a revolution—a melding of other styles of genre storytelling that became the model for what we now think of as the “Golden Age” of TV.

As fans remember the show on the 20th anniversary of its launch, Buffy is being lauded for the vitality of its female characters, its clever mixing of fantasy and horror tropes with regular teen anxieties, and the radical empathy of its characterizations. But Joss Whedon’s show also set the framework that many of the Emmy-winning prestige dramas of later years, from The Sopranos to Breaking Bad to The Good Wife, would use to tell their stories. Years before streaming TV existed, Whedon helped create the bingeable serial drama—one that endeavored to make every episode a special event, without taking the audience’s eyes off the larger story being woven.

In basic terms, he did this by making sure every season had a “big bad”: a villain or antihero with larger machinations developing in the background of every episode, twinned to our hero Buffy and her resolute band of friends in some magical way. Every season would build to an action-packed climax with sacrifices made and lessons learned, but along the way, Buffy would face off against minions of the “big bad,” problems of her own making, and various other monsters of the week amid whirlwinds of teen angst. It was a heady formula, but a surprisingly unusual one for 1997.

Of course, serial storytelling had existed in broadcast television since the invention of the medium—mostly confined to soap operas. In the late ’70s, the primetime soap Dallas popularized the season-ending cliffhanger as a way of keeping audiences on the hook and building buzz over its summer break; in the ’80s, imitators like Dynasty, Knots Landing, and Falcon Crest dominated in the ratings, while critically acclaimed procedural dramas like Hill Street Blues (a cop show) and St. Elsewhere (a hospital show) incorporated elements of longer-arc storytelling along with the usual cases of the week.

David Lynch and Mark Frost’s 1990 phenomenon Twin Peaks, too, was driven by its serial nature—viewers tuned in largely because of the enduring mystery of who killed Laura Palmer—but couldn’t sustain it. It strung the mystery along as far as it could (Lynch later noted that he never wanted it solved, but network ABC insisted), and then tanked in the ratings after trying to move on from it. Sci-fi dramas like Star Trek: The Next Generation strongly avoided heavy serialization for fear of confusing viewers, whereas the cult hit Babylon 5 (which debuted in 1994) was practically unwatchable unless you’d seen every episode, granting it a devoted, but small, fan base. The almost-forgotten pioneer Murder One, a legal drama that tackled one big case per season, was acclaimed when it debuted in 1995, but audiences and critics alike were not interested in the new story offered in its second season, and it was canceled.

One of the biggest influences on Buffy’s storytelling style (and the shows that followed it) was The X-Files, famed for its ongoing mythology. But that show took careful pains to separate “monster of the week” episodes (which could vary wildly in tone and format) from the ongoing “mytharc” (which was, again, basically impenetrable unless you were a devoted viewer). With Buffy the Vampire Slayer, Joss Whedon cracked the code on how to combine the two, by borrowing from the comic book realm he’d later inhabit as a film director.

Each episode of Buffy can be funny, terrifying, action-packed, high fantasy, or a heady mix of it all; they’re all memorable, even the ones Whedon would probably rather forget (season four’s “Beer Bad” always comes to mind). Some of them only vaguely mention the overarching villain of a season, while others are intrinsically concerned with them; there might be ongoing subplots at work too, involving the characters’ various romantic entanglements or lesser villains, from season two’s Spike and Drusilla to season six’s nerdy Trio. Like a comic book, you could pick up an issue and enjoy the story without worrying too much about the larger, overarching plots, but there was usually enough about them to get you interested in reading the next one.

That mix—that emphasis on serialized storytelling that still revered the individual episode—is the balance struck by so many of the greatest TV shows in recent memory. It’s something that echoed through TV sci-fi and fantasy (with shows like Battlestar Galactica and Lost) but quickly transcended it. That narrative mix feels as important to the development of weighty dramas like Mad Men and The Shield as it does to frothier works like 24 or Veronica Mars. And it’s something that Whedon later brought to the world of studio movies, writing and directing The Avengers for Marvel and showing how individual films can feel like episodes in a larger epic series, drawing in one-time viewers and hooking them as they tease the next installment.

It’s also a model that’s increasingly vanished from the world of television. Streaming companies like Netflix, which has helped sustain Buffy for viewers who were born after it first aired, are presenting their own shows modeled more on HBO’s similarly revolutionary The Wire—novelistic tales in which each episode is just a part of a greater whole, and an entire season tells one story. In an era where people no longer discover TV shows by flicking between channels, there’s no longer any commercial necessity to strike a balance between episodic and season-long narratives.

Whedon himself has acknowledged how different things have become. Asked for his thoughts on binge-watching, he recently told The Hollywood Reporter: “The more we make things granular and less complete, the more it becomes lifestyle instead of experience. It becomes ambient. It loses its power, and we lose something with it. We lose our understanding of narrative. Which is what we come to television for.” For him, part of the power of Buffy’s episodes was about bringing audiences together on a weekly basis—“I would want people to come back every week and have the experience of watching something at the same time,” he said. Buffy, in 1997, harnessed that power better than any show before it, and helped evolve it into something new. It might feel like a TV relic now, but its storytelling DNA is now inescapably baked into the medium.