Notes On A Scandal (Richard Eyre): Judi Dench, Cate Blanchett, Bill Nighy. Running time: 98 minutes . . .

TEENAGE boys like to brag. They also like to dream lusty thoughts about unobtainable teachers. It's an eternal mystery then that the 15-year-old youngster who seduces the thirtysomething blonde in Richard Eyre's latest film keeps his cool. After all, he's just bagged A-list actress Cate Blanchett. You would at least expect him to tell a few mates or crack a fire alarm in delight. But the most excitable he gets is playing football. He takes his shirt off in the playground to celebrate a goal and the onlooking art teacher Sheba Hart (Blanchett) puts a hand to her neck and blushes.

You know right there and then that this is going to end in afterschool detention - the kind that involves one-to-one tuition and a just-swept-clean desk.

Blanchett and young Derry actor Andrew Simpson share one of the raunchiest kisses on the screen this year - their extracurricular activities would fog a classroom window judging by the messages he leaves on her phone.

Notes On A Scandal is a drama about sexual obsession and middle-class female dysfunction based on the Zoe Heller novel.

Judi Dench, who also starred in Eyre's previous film Iris, delivers a pitch-perfect performance. She plays an in-the-closet lesbian teacher who discovers the affair and decides to wrangle something out of it for herself - the affections of Ms Sheba no less.

Here's a woman so bitter with life, she would curdle the cream in your coffee. Dench and Blanchett (both are Oscar nominated for these performances) work up a hothouse of tension.

But their moment of reckoning - the point when the sparks should fireball into a satisfactory bang - is rendered impotent by a plot that lets all their hard work dissipate into thin air. And so Notes On A Scandal goes out with a whimper.

Judi Dench's elderly teacher Barbara Covett is a lonely spinster with a sour grimace crumpled permanently on her face.

She keeps a cat ("someone has died" she announces when it is put to sleep) but she has no friends and certainly nothing good to say about anyone. She keeps a diary for company and in it she pours nasty, snobby observations, some of which are narrated in voiceover. Her working-class students are "pubescent proles". When Sheba invites her over for Sunday lunch, her poisoned pen describes the host's older husband (Bill Nighy) as a "crumbling patriarch" and their son who has Down Syndrome as "the court jester".

Dench narrates the story with a nasty glee, adding not a little literary flair - each syllable is rounded in that plummy palate before being spat out with a coating of acid.

She strikes up a reserved friendship with Sheba ("a wispy novice? a fey person I suspect, " Barbara tells us) but is drawn to her naïve honesty. Sheba has begun teaching pottery at the school after spending many years looking after her disabled son. She is beautiful, has a nice family and a comfortable life. But she is also having sex with a 15year-old boy and thrives on the danger. Blanchett plays her with a detached, foolish air. She is evidently going through an emotional meltdown. In one of the film's many black moments, her young paramour asks, "Can I smoke miss?" They have just finished rolling around the tracks at a train yard.

When Barbara discovers the affair, she doesn't tell the police on the condition that Sheba tell her husband after Christmas.

She imposes a suffocating, supportive friendship and starts turning up uninvited. Then a comforting hand rub is offered in the kitchen.

"No one can violate our magnificent complicity, " Barbara tells us in voiceover. She starts to turn the screw. "She's the one I've waited for."

The film brings to mind Strangers On A Train - Hitchcock's masterly study in homosexual obsession. But Notes On A Scandal lacks that lurking psychotic danger; certainly, it has none of the tinge of guilt. Eyre's film actually works to avoid confrontation: Dench and Blanchett earn their showdown, but Patrick Marber's script prefers to have matters dealt with by the law, in the kind of fashion you read about in the papers.

It seems more comfortable as a study of female pathology - the psychosexual, but without the psycho - and so ends up reflecting the very middle-class standards Barbara so effectively lampoons in her diary throughout the film.

Still, Judi Dench is terrific. She is now in her early 70s and just keeps getting better.