The aptly named Barbara Covett (Judi Dench) is a self-described battleax who's taught history at a low-rent London school for about four hundred years. Close to retirement, she lives in a dusty flat with a vomiting cat as her only company and has nothing but disdain for everyone that crosses her path. She thinks her colleagues at school are morons, and appraises the student body as "future shop assistants and plumbers".

Barbara religiously records her every thought and experience in the diaries she has kept since childhood, and we hear a lot of it in Dench's clipped voiceover throughout the film. Though she doesn't have a positive word to say about anyone or anything, she doesn't miss a trick: Barbara's completely obsessed with everything around her, and though her outlook is jaded in the extreme, her sociopathic desolation has driven her to maniacally covet the affairs of others.

So, when the Bohemian Sheba Hart (Cate Blanchett) arrives in the staff room as the new Art teacher, Barbara has a new fish to brain-fry. "Is she a Sphinx or simply stupid?" Barbara asks as she watches Sheba perform playground duty and struggle with classroom riots. When Barbara marches into one of frazzled Sheba's classroom fracas and instantly petrifies the tearaway pupils into obedient silence, the two women begin a tentative friendship when a greatful Sheba invites Barbara first to coffee, then to her home for Sunday roast.

The sad old thing responds to Sheba's invitation with feigned ambivalence but privately, with utter rapture. Proclaiming the day "a Gold Star day", she spends the next couple of days getting her hair done, buying new clothes and shoes as if she had an upcoming audience with the Queen (if only Dench put as much effort into selecting her Oscar frocks, which are invariably drab)

Barbara's surprised on numerous counts when she arrives at Sheba's for lunch. Answering the door (to a fairly nice house in what Barbara calls "bourgeois Bohemia") is Sheba's much older husband (Bill Nighy), while inside lurks a chain-smoking slutty teenage daughter and a ten-year-old Down's Syndrome son, Ben.

Barbara hates Sheba's cooking and is contemptuous of the entire family, but remains elated that she has made a "friend" in Sheba. Despite Barbara's delusions, her and Sheba have nothing in common and the sociable Sheba would probably invite anyone and everyone over for lunch. After a few wines, Sheba opens up to Barbara and speaks candidly about her troubled relationship with her mother, her confidence doubts about teaching, her efforts raising Ben. Barbara listens attentively, convinced Sheba's revelations have cemented their mutual trust and friendship.

Barbara's idea of friendship is that it should be a strictly one-on-one affair, so things become unstuck in a big way when - peering through a classroom window - she catches Sheba giving head to 15-year old Steven Connolly (Andrew Simpson), a non-academic hottie whose talent for drawing won him after-school art lessons with Sheba. Barbara confronts Sheba immediately, but connives to not tell anyone about Sheba's illegal transgression as she will then have Sheba locked in her debt and therefore a "friendship" for all eternity. Greatful Sheba agrees to Barbara's demand to end the affair immediately, but Sheba has fallen in love both with Steven and her lost youth and independence ("I hadn't been pursued like that in years", she tells Barbara). Behind Barbara's back, Sheba continues her wild sexual affair with Steven.

The plot thickens further when it turns out that Barbara's friendship with Sheba may not be platonic. Sheba rejects Barbara's ginger attempts at physical intimacy which embarrasses and offends Barbara. Sheba cops strike two when Barbara learns that Sheba is still fucking Steven. Barbra tells Sheba off, but gives her one last chance. After all, according to Barbara, Sheba is the one Barbara has waited for her "whole life".

Strike three comes when Barbara's cat has to be put down, and she rushes to Sheba's house in a state of complete devastation. But at that moment, Sheba is leaving with her family to watch Ben perform in his first school play: she can't stay around and console Barbara. For Barbara, this is the ultimate violation of her idea of friendship, and as she tells Sheba, "you owe me!" and even threatens to spill the beans to Sheba's nearby family. Later that night, Barbara betrays Sheba and the scandal of the title is unleashed.

This deliciously titillating plotline, from the novel by Zoe Heller, could have been a trashy season-long story thread of Eastenders but Notes on a Scandal is a sophisticated, superlative film which treats us to two of the greatest film actresses slugging it out in one well written, dazzling acted scene after the next.

Dench gives her finest performance, and that's certainly saying something. Every nuance of face, voice and movement is meticulous, perfect. Her performance is a devastating portrait of the bitter loneliness of the lifelong anti-social who seems to crave company but who lacks the warmth to bring anyone into their life. Almost seventy years of what Barbara terms her unremitting, "dripping tap" solitude have turned her sociopathic, quasi-psychotic. Dench's rendering of Barbara's malevolent humanity is masterful. Maybe only Bette Davis could have done better.

The imperious Blanchett keeps her epic charisma in check right up until the climactic scenes. She's thoroughly convincing as a middle-class mother of two trying to find some new spark in a life that's turned stagnant.

(Does anyone else note the multiplicity of similarity between Cate Blanchett and Kate Bush? Despite their same first names, (whose different first initials could stand for Comparable Kates), they physically resemble each other, remain aloof from the tabloid hoopla of their respective industries, and share the same kind of elysian artistic genius.)

Special mention too to handsome Andrew Simpson, whose hormone-crazed Steven holds his own in what must have been the intimidating company of his superwomen co-stars.