Twenty years ago this month, the Hellmouth opened, and television hasn’t been the same since.

On March 10 in that long-ago year, an upstart network called WB broadcast the first episode of “Buffy the Vampire Slayer,” Joss Whedon’s smart, campy, funny series about a teenage girl who is the world’s best defense against all sorts of demonic forces.

Five years earlier, a movie of the same name written by Mr. Whedon hadn’t made much of an impression, but the TV show, with Sarah Michelle Gellar in the title role and a perfectly cast collection of supporting characters, became a phenomenon.

With its girl-power message and its deft mix of horror, humor and romance, “Buffy” quickly developed a passionate fan base. Teenagers watched it. College students developed drinking games around it. Tweener girls and their mothers made it the kind of bonding ritual later identified with “Gilmore Girls.”