“Notes on a Scandal”Fox Searchlight PicturesDirected by Richard EyreWritten by Patrick Marber, based on the novel by Zoe HellerStarring: Judi Dench, Cate Blanchett, Andrew Simpson and Bill NighyRated R/91 minutesOpens January 12, 2007Three-and-a-half out of four stars

Anyone who’s ever enjoyed those nature videos in which the black widow spider lures the sweet, unsuspecting fly into a web of doom will especially like Judi Dench’s deliciously vampirific performance in Richard Eyre’s “Notes on a Scandal,” one of those edge-of-your-seat melodramas about psychotic stalkers that used to be all the rage in the early ’90s. Yeah, so it’s kind of trashy, but in a cheeky, classy way.

Dench plays Barbara Covett, a 60-ish history teacher at St. George’s School in London who probably grew up more focused on her studies than on the lesser mortals around her. All work and no play has made Barbara a lonely, bitter woman, warily respected at her school, but with no one to tell her secrets to but an almost dead cat and a diary (see where a strict work ethic and a college education gets you, boys and girls? Drop out now while you still can).

It’s in this diary that Barbara writes her cynical observations of the world. About the rambunctious boys at her school she writes, “In the old days, we confiscated cigarettes and wank mags. Now it’s knives and crack cocaine. And they call it progress.” Even when she’s writing about Sheba Hart (Cate Blanchett), the new pottery teacher who’s pretty in an unassuming, unfussy, bohemian way, Barbara’s first reaction is to be snobbish (“Artfully disheveled today”), but it’s only her way of arrogantly diminishing the world to a size that she can comfortably enter.

Barbara likes Sheba and forces an intimate friendship between the two of them when she discovers Sheba canoodling with a 15-year-old student (Andrew Simpson). Dench is, of course, stellar at evoking Barbara’s bipolar moods-the instant Sheba can’t be there for her, Barbara turns vile, threatening to reveal the secret.

Why Sheba would risk destroying her family and her marriage to a loving husband (Bill Nighy) is unclear-maybe she’s frustrated with raising her children or is no longer enticed by a husband who is probably twice her age (maybe she saw Nighy play Davy Jones in “Pirates of the Caribbean” and got grossed out at the idea of kissing a dude with tentacles on his face). The fact that Patrick Marber’s script, based on Zoe Heller’s novel, doesn’t spell this out shows respect for our intelligence.

You could say the ending descends into silliness, but I’d like to say it ascends to it. There’s a lot of screaming, nail-clawing and running mascara in the final moments when the worst possible person stumbles across Barbara’s diary. The hysteria reaches giddy, operatic proportions and at that point the movie’s real identity-as a well-made, well-acted psycho-thriller-comes full blossom. When Barbara settles into that park bench at the end and sets her sights on her next victim, it’s unsettling in a kind of darkly humorous way.

Another fly for the spider.

“Oh, how I do like them young and hairless.” Cate Blanchett does the Mary Kay Letourneau thing in “Notes on a Scandal.”