“If the apocalypse comes, beep me.”

Twenty years on, the brilliance of Joss Whedon’s funny, tragic seven-season synthesis of most film and TV genres, literary conceits and pop cultural tropes remains undimmed.

Buffy Summers’ apocalypse-averting adventures in Sunnydale began in 1997. The show was based on a film not many people saw and which got mostly bad reviews from those who did. Its writer, Whedon, who had watched in despair as his vision was macerated by the movie-making process, got a second bite at the creative cherry when he was given the chance to turn his high-school-is-literally-hell concept into a TV series.

The one-in-every-generation girl chosen by fate to be the vampire slayer and fulfil her world-saving destiny … Buffy. Photograph: Allstar/20th Century Fox/Sportsphoto Ltd

Inverting the traditional horror movie staple, the pretty blonde high school student didn’t die in the opening credits but turned out to be the enduring heroine – the one-in-every-generation girl chosen by fate to be the vampire slayer and brought to the Hellmouth (atop which Sunnydale High sits) to fulfil her world-saving destiny. She is helped by her friends, collectively known as “the Scooby gang”, in just one of many intertextual details that helped make the show as loved by academics as by viewers – geeky Willow (Alyson Hannigan), hopelessly uncool Xander (Nicholas Brendon), effortlessly cool Oz (Seth Green), former demon Anya (Emma Caulfield) – and her lover and vampire-with-a-soul, Angel (David Boreanaz). She is hindered by Spike and Drusilla (vampires very much without souls, the Sid and Nancy of the undead), a different monster every week and a Big Bad offering an overarching threat each season.

You might have come for the genre storytelling, but you soon found yourself staying for so much else. The feminist sensibility. The funny. The slang that worked so well and felt so right that it got its own name – Buffyspeak. The deep-laid plot lines. The guts. The grief. The glory. The dense allusiveness of the script. It paid to watch and rewatch then and now, two decades later.

When Angel rejects Buffy, teenage girls everywhere nodded, sighed, and wished that they too could simply stake their first loves through the heart. Photograph: Allstar/20th Century Fox/Sportsphoto Ltd

Everywhere was thickly strewn but lightly handled metaphor. While the vampires stood for all they have ever stood for – rebellion, subversion, predation and sexuality – Buffy, by virtue of her slayer status, was the perennial outsider, a walking (“in stylish yet affordable boots”) embodiment of teenage alienation. Individual episodes dramatised particular adolescent fears. In Witch, a domineering mother literally takes over her daughter’s life via a bodyswitch. In Family, timid Tara is nearly taken out of college by her father and brothers who aver that their demonic heritage means all the women in their family become evil when they reach adulthood – it turns out to be a myth perpetrated by the men of the family down the generations to keep the women passive. And when Buffy sleeps with her ensouled-vampire boyfriend, he turns into a different person overnight and rejects her the next morning. In his case it was because of a gypsy curse that de-souls him, but teenage girls everywhere nodded sagely, sighed, and wished they too could simply stake their first loves through the heart. At the end of season two, Buffy “comes out” as a slayer to her mother, who asks her if there is any way she could stop being one and throws her out when she says she can’t. In later series, the gay subtext is made text via a relationship between Willow and Tara, still one of the most positive, remarkable-in-its-unremarkability and, alas, rare depictions of young lesbian love around.

Over the seven seasons, things become progressively darker and more ambitious as the Scooby gang grows up, leaves school and heads for college and beyond. Buffy grows into her slayer role as ordinary teen mortals grow into adulthood, negotiating the new complexities, resenting the new responsibilities and eventually – if only in the absence of an alternative – accepting them.

Spike’s chip, the temptations offered by growing power (Slayer or Wiccan), the effect of grief and rage upon a person all posed so many questions of identity, morality and responsibility that if the propulsive storytelling or snappy one-liners had ever let up you would have collapsed under the weight of the philosophical complexity by the time the credits rolled. Fortunately, they never (or almost never – the seventh season had its tottery moments) did. Viewers revelled in the embarrassment of riches offered, and a whole academic discipline arose (Buffy studies) to unpack it all at leisure elsewhere.

If the snappy one-liners had ever let up you would have collapsed under the weight of the philosophical complexity … Buffy. Photograph: Sky TV

The Sopranos is generally held up as the inflection point for television-as-art – the moment the medium matured and had to start being taken seriously. But Buffy was there first and doing extraordinary things before the conflicted Mafiosi hit the screen – and long before Six Feet Under, The Wire, or Battlestar Galactica. It had episodes of pure comedy, like Band Candy (in which the adults of Sunnydale revert to their youth under the influence of enchanted sweets), high drama (The Becoming, in which Buffy sacrifices Angel to close the Hellmouth) and aching realism (The Body, where Buffy’s mother Joyce dies of natural causes – leaving Buffy, for all her special strengths, powerless to right this single most terrible wrong).

On top of these there were episodes like Hush, which was almost devoid of dialogue and still managed to be hilarious and utterly terrifying (while also asking questions of what remains if you take language from us) and Once More With Feeling, a full-blown musical that left you breathless with delight and disbelief at what had been revealed by the end.

Under cover of a tiny network (WB), a young audience and po-mo-gothic darkness it came and gave us everything – laughs, tears, epic sweeps, tiny moments, comedy, drama, realism and surrealism – without a missed beat or a wasted word. It made young female characters central without sidelining the male in a way not seen before or – at least with such complexity, such care – since. It fed your heart, your soul, your mind. Several TV generations have elapsed since it ended, but we have not yet seen the Slayer’s deft, clever, moving, thrilling, funny, feminist like again. Buffy still stands alone.