Tangled webs: "Spider-Man" and "The Lady and the Duke."(two motion pictures)

What happens to movies? Whom should we blame when a big studio picture goes astray, or does it have an entropy of its own? To be specific: how come "Spider-Man" begins in such good humor and ends, like Eensy-Weensy, completely up the spout? The opening credits throw themselves into position with an off-kilter, nineteen-fifties brio; the Saul Bass who put together the title sequence of "Vertigo" would have looked on with approval. Then we are tipped straight into Peter Parker (Tobey Maguire). "This story, like all stories worth telling," he informs us, "is all about a girl." True enough, especially when the girl in question, Mary Jane, is played by Kirsten Dunst, who gives a dreamy, near-perfect impersonation of someone who really doesn't mind having to dye her hair red for the sake of a movie.

Peter is a supergeek. This we know because he wears spectacles, runs for the bus, and aces science. In short, he is crying out for an alternative personality. (So many comic-book heroes turn out to be extended riffs on the before-and-after promise of a Charles Atlas advertisement.) His wish is granted when a genetically modified spider nips him on the hand during a school trip; back home, where he lives with his uncle (Cliff Robertson) and aunt (Rosemary Harris), Peter enjoys a refreshing sleep and wakes up with an improved physique, sharpened vision, and an overwhelming desire to clamber out of people's drains when they're about to run a bath.

There is, it emerges, nothing disgusting about becoming an arachnanthrope. Peter fails, for instance, to sprout a further quartet of limbs or a funky tarantula fuzz on his upper thighs. His cutest new knack is for ejecting long, twangy threads of sticky stuff from his wrists; there is, perhaps, a happy touch of Portnoy to this unfamiliar gift, and I think that Maguire is wise to it. Certainly, the smartest scene in the movie comes when, full of the joys of his spring, he leaps onto a roof and tries, for the first time, to shoot his silk into the void. "Go, web, go!" the desperate teen-ager cries, madly flipping his hand back and forth. Hmmm. Maguire is much the best thing about the film. The more those millions of comic-book addicts (Where are they all? Do we get to see them in daylight?) fretted online that he might be wrong for the part, the more I knew that he was right, not least because it is the essence of Peter to worry whether he deserves the role of hero. The glum-nervy smile, the sidelong glance of the slightly bug eyes, that modest arching of the brows: all the Maguire tics were ready to roll, and there is something absurdly winning in Peter's expression as he realizes what heights he is now at liberty to scale. With so much in prospect, he is almost afraid of himself.

The trouble with the older comic-book ethics is that they have a simple, thudding solution to this dilemma: civic duty. As Peter's uncle puts it, "With great power comes great responsibility." Not so. With great power comes a great ability to check out the view from the top of the Chrysler Building without having to wait for the elevator. The director of "Spider-Man," Sam Raimi, has enough B-movie serum in his bloodstream to know what manner of kicks we want from this picture; we want to see Peter swing down the avenues on long, looping arcs of thread, a trapeze of his own devising, like a flying Sinatra. It may be the hippest method ever devised for beating traffic. No wonder you can feel Raimi losing interest in the project, if not losing the plot, as Spider-Man is forced to quit horsing around and start battling for the soul of New York. Worse still, a nemesis arrives in the form of a rich scientist named Norman Osborn (Willem Dafoe), who suffers a regrettable incident in his laboratory involving Oz-style smoke. From this he emerges as the Green Goblin, an airborne lump of id in a safety helmet. "Forty thousand years of human evolution and we've barely even tapped the vastness ...