I have not yet heard a convincing explanation for the general upsurge in popularity of vampire films and TV series in the last few years. But the particular success of the Twilight franchise, based on the mega-bestselling series of young adult novels by Mormon writer Stephenie Meyer, seems less of a mystery. Despite handicaps like wooden performances and clumsy dialogue, the films are as perfectly attuned to the sensibilities and yearnings of young adolescent girls - and at least as politically incorrect and socially retrograde - as today's violent video-game-based action blockbusters are for pubescent boys.
The heroine, Bella (Kristen Stewart), is a moody, tricky and self-obsessed teenager who has fallen in love with a high-school classmate. He is spectacularly handsome in a Byronically pale and gloomy way, with dark red lips, strange orange eyes and remarkably thick eyebrows, and he happens to be a vampire - albeit a civilized and modern kind of undead bloodsucker whose benign clan choose to feed on animals rather than humans.

(Unlike other movie vampires, this lot do not inhabit coffins and they can survive in daylight, though they have chosen to live in a perpetually overcast American Northwest.)

Edward, played by British heartthrob Robert Pattinson with a flawless American accent, is at least as in love with Bella as she is with him. Being more than 100 years old, he knows his own heart, is comfortable in his own sparkly white skin, and is as protective as he is passionate. Moreover, he believes in fidelity, monogamy and true love, a combination arguably
as rare in real-life teenage boys as vampirism.

Quite why this fantasy figure brought to life should be so excited by Bella and so devoted to her is one of the enigmas of the franchise. Stewart, who has given some good performances in the past in films like Adventureland, makes Bella a morose, mumbly, unsatisfied creature whose moods swing from anguish to despair and back.

When the film opens, the lovebirds are lying in a field of lavender and looking into each others' eyes, as she recites Robert Frost's poem, Fire and Ice. Bella is desperate to be "turned" - into a vampire - by Edward. He feels guilty about the idea of shortening her human life before she has had a chance to live it as an adult, and says he will only turn her if she marries him first.

The hatred they feel for each other has a special, almost kinky intensity

The same goes for sex, partly because vampires in their excitement can be lethally dangerous to human lovers, and partly because, as a man born in the 19th century, he believes in chastity before marriage.

Bella keeps his proposal a secret. Her father, a hard-drinking divorced policeman, does not like Edward. More importantly, Bella is fascinated by another hunk - the muscled, perpetually shirtless Jacob who belongs to a wholesome local Indian tribe who are all werewolves. Jacob (Taylor Lautner) is as earthy and masculine as Edward is anguished and poetic, and is, of course, passionately in love with Bella.

Knowing all about the vampire presence in the area, and about her relationship with Edward, Jacob is violently determined to win her heart and to rescue her from an undead future. The warm-blooded werewolves are mortal enemies of the vampires, but the hatred Jacob and Edward feel for each other has a special, almost kinky, intensity.

As Bella steps between the two of them to stop them coming to blows or bites, you cannot help thinking that she rather enjoys being at the apex of an erotic triangle.

However, the relative peace of their mountain town is threatened by the imminent arrival of a murderous renegade vampire leader from Seattle with an army of "new born" super-strong vampires.

The renegades have one aim - to kill Bella (everything in this convincingly adolescent universe is all about her.) Edward's vampire clan and Jacob's werewolf tribe will have to work together to defeat the invaders and protect her.

It is understandable that millions of teenaged and "tween" girls would enoy the idea of a rather ordinary girl who is pursued by two extraordinarily handsome boys.

But what makes the Twilight series a kind of emotional porn is the way that neither of the boys are pushy for (lethally dangerous) sex but simply want to be with the heroine always (literally in the case of the vampire boy), and that the boy she loves the most is actually a mature, generous, attentive and much older man in a teenager's body.

I am told by those who have seen all three films in the series that this latest is vastly superior to its predecessors. The movie's action sequences, in particular the battles between vampires and werewolves, are presumably intended to broaden the series' appeal to males for whom intense romantic virginal yearning does not do the trick (this was presumably why the producers hired director David Slade who made the horror movie 30 Days of Night for this film).

There are welcome hints here and there that the movie does not take itself too seriously, and some of its more amusing lines by screenwriter Melissa Rosenberg are clearly intentionally funny.

And in the end, despite weak acting, some thudding dialogue, and fake-looking mountain scenery Twilight: Eclipse turns out to be strangely, surprisingly watchable.