Sarah Michelle Gellar’s just signed up to reprise her “Cruel Intentions” role in NBC’s sequel to the 1999 cult hit film — in which her wealthy, diabolical character plots the demise of the innocent Reese Witherspoon.

And Gellar really, really needs for this show (set more than 15 years after the movie) to work because her career’s in a downslide.

When “Buffy the Vampire Slayer” ended in 2003, Gellar was at the top of the heap as the lead on a pop-feminist phenomenon that earned both a hugely loyal following and critical love. Creator Joss Whedon could do no wrong — even pulling off a great musical episode! — and Gellar benefited.

But Gellar’s been Hollywood kryptonite ever since, with the flops “The Crazy Ones” (a sitcom dud where she played Robin Williams’ daughter) and “Ringer” (a soapy action dud where she played a set of twins). As for her movie career, let’s just say it’s not quite as impressive as that of the Oscar-winning Witherspoon.

Going back to “Cruel Intentions” — in which her smooch with Selma Blair won the MTV Movie Award for best kiss — may look like a retread for Gellar, but it could also be her smartest move in years.

Now 38, she’s still struggling to establish herself as a bona fide, full-fledged woman in the eyes of Hollywood, which has clung to her image as teen-bait for way too long. Even when she played a real adult, it rang false — Gellar’s not a technically dazzling actress and needs the writing to be exactly tuned to her strengths, something Whedon always nailed.

Another one besides Whedon who got her was the writer-director of “Cruel Intentions,” Roger Kumble, who hit that sweet spot of flighty pop attitude and style. The great news is that he’s behind the NBC pilot. This. May. Just. Work!

If we can’t have Gellar and Whedon team up again — and some of us will wait forever, or until he gets that spandex-superhero nonsense out of his system, whichever comes first — this could be a pretty nifty alternative.