Eighteen years ago, Buffy the Vampire Slayer aired for the first time on The WB in a two-part debut. “Welcome to the Hellmouth” introduced viewers to Buffy Summers (Sarah Michelle Gellar), a likable, popular 16-year-old who just happened to have a destiny that included saving the world from the undead; while also introducing Buffy to Sunnydale, a small, run-of-the-mill California town that just happened to sit on a Hellmouth—a portal of mystical energy that attracts demons, vampires, and other boogeymen.

In many ways, Buffy was a conventional television heroine, in that she was pretty and blonde and perky, and she talked a mile a minute about clothes and homework like a normal teenage girl (as opposed to the theatrical soliloquies delivered by characters in The WB’s next teen drama, Dawson’s Creek). But the show’s subversiveness was spelled out by creator Joss Whedon in the opening scene, as a nervous blonde girl and a cocky, older-looking guy in a leather jacket break into Sunnydale High School late at night. The girl seems scared and keeps hearing noises. “We’re just going to get in trouble,” she tells him. “You can count on it,” he replies, licking his lips. Finally, once she’s certain there’s no one else around, she reveals her distorted vampire features and sinks her teeth into his neck.

In its first two minutes, “Welcome to the Hellmouth” establishes a universe, as the camera creeps slowly through a high school in the dark, lingering on skeletons in a classroom and shadows behind a door. But it also establishes a premise—that this is a show about female power. The pretty blonde, a vampire named Darla, isn’t a victim but a predator, just as Buffy has strength and acuity that belie her looks. At the end of “The Harvest,” the second part of the two-episode debut, Giles (Anthony Stewart Head) watches Buffy and her friends walk away, cheerily discussing the ways in which she stands between the world and its total destruction. “The earth is doomed,” he says, wearily. It's this kind of assumption—that being young and frivolous and having profound influence are mutually exclusive—that Whedon would go on to dispel throughout Buffy’s seven seasons.

Buffy is also unique in that, as much as Sunnydale is a Hellmouth, high school is hell. The monsters in the first season are literal manifestations of the demons that plague teenagers—a controlling mother who's so intent on living through her daughter that she switches bodies with her, a pack of teenage boys who become uncontrollably feral, an Internet boyfriend who pretends to be a normal kid but whose real identity is much more complex (in this case, he's a demon called Moloch who got accidentally uploaded onto the Internet). In the episode “Out of Mind, Out of Sight,” one of Buffy’s classmates is ignored to the extent that she physically becomes invisible. In transmogrifying common teenage issues into actual, tangible villains, Buffy makes them seem important, and worth agonizing over. It also makes them seem conquerable.

This sense of empathy for the complexity and horror of the high-school experience comes through as Buffy’s characters defy their stereotypes to become fully realized people, with help from the Slayer. Willow, pigeonholed as a nerd, becomes confident and powerful (too powerful) throughout the course of the show, while Cordelia, the shallow and cruel Queen Bee, becomes much more sympathetic. But Buffy herself arrives in the pilot fully-formed—smart, kind, sensitive, and aware of what she wants in life, which is mostly not to have to kill vampires every night and to just be a normal teenager. What she realizes along with the audience during the first season is that there’s no such thing. Every kid has her own demons to slay—some just have pointier teeth than others.

Television has had lots of complex, admirable teenage heroines since Buffy, but it's hard to think of one so consistently empowered to take control of the circumstances around her, whether in the middle of a graveyard, surrounded by vampires, or in her bedroom, grounded. Buffy’s super-strength is a physical attribute endowed by the forces of destiny, but it's also a state of confidence and competence that carries her through the varying traumas of having the fate of the world always on her shoulders. Joss Whedon’s 2006 speech after being honored by the group Equality Now describes his frustration at consistently being asked why he writes strong female heroines. The real reason, he finally states, is because people are "still asking me the question.” In a perfect world, Buffy at 18 wouldn't still seem so revolutionary, but if nothing else, it's another reason to appreciate it.