By JANET MASLIN

The funny thing about Harry Potter is that he was famous from the start. "There will be books written about Harry -- every child in our world will know his name!" J. K. Rowling announced with spooky accuracy in the opening chapter of her first novel, "Harry Potter and the Sorcerer's Stone." No doubt she meant this as a reflection of Harry's awesome powers of wizardry, not of his ability to land on magazine covers and lure children to bookstores at the witching hour. But the fact is that Ms. Rowling's gifts of prophecy have proved nearly as amazing as all the magical feats she ascribes to Harry and company. Still, not even she could have predicted this.

The frenzy that has greeted the fourth book in the series, "Harry Potter and the Goblet of Fire," would seem to go beyond any reasonable response to fiction, no matter how genuinely delightful that fiction happens to be. Instead, the current wave of Harrymania brings the Potter series to a fever pitch better associated with movie hype, major sports events and hot new Christmas toys. And it has placed Ms. Rowling, already the golden goose of publishing, in the dicey position of having to outdo herself with a sequel written under huge commercial pressure at twice the length of any of her previous books. We know what happens to talent when the tie-ins hit the Happy Meals, and the stakes get this high.

But Ms. Rowling, a kindred spirit to both Lewis Carroll and the pre-Jar Jar Binks George Lucas, turns out to be a fantasist who lives inside a thrillingly fertile imagination, mines it ingeniously and plays entirely by her own rules. Talk about supernatural tricks: she has turned this odds-defying new book into everything it promised to be. As the midpoint in a projected seven-book series, "Goblet of Fire" is exactly the big, clever, vibrant, tremendously assured installment that gives shape and direction to the whole undertaking and still somehow preserves the material's enchanting innocence. This time Ms. Rowling offers her clearest proof yet of what should have been wonderfully obvious: what makes the Potter books so popular is the radically simple fact that they're so good.

It is not immediately clear that "Goblet of Fire" is a step forward for the series, since it gets off to a shaky opening. Displaying her only real Achilles' heel, Ms. Rowling starts the book with a sinister, tacked-on prologue hinting at the whereabouts of evil Lord Voldemort, who is the Darth Vader of this enterprise and is so wicked that others fearfully refer to him as "You-Know-Who."

When she cuts cinematically from this whiff of peril to Harry's awakening with a start, she resorts to the kind of predictable storytelling signals that her narrative doesn't need. When you can dream up an idea like the Pensieve, a basin to hold one's excess thoughts until they can be dealt with at leisure, or a household clock that indicates family members' whereabouts ("home," "work," "traveling," "prison," "mortal peril") instead of the mere time, there's no excuse for falling back on the humdrum.

The new book starts off with some explaining to do (and handles it expeditiously enough to be inviting and accessible to first-time Potter readers). Then like the three volumes before it it rescues Harry from his miserable foster parents (who this time will send him a single tissue as a Christmas gift) and restores him to the Weasley family, where he is warmly welcomed and where Mrs. Weasley cooks dinner with the help of magic ("pointing her wand a little more vigorously than she had intended at a pile of potatoes in the sink, which shot out of their skins so fast that they ricocheted off the walls and ceiling").

Harry and most of the Weasley children are headed for Hogwarts, the academy of magic that Ms. Rowling has turned into the series' most seductive attraction, what with mischievous, mobile portraits on the walls and studies like Potions and Care of Magical Creatures. Never has one author done so much to make readers of all ages long to be at school.

But en route to Hogwarts, in a book that owes its expansive length to many such alluring diversions, the Weasleys and Harry stop at a kind of wizards' Woodstock. This huge outdoor event, at which magical types pitch wildly fanciful tents but can't figure out how to light matches, revolves around the World Cup Competition in the game of Quidditch, which would by now be played on sandlots and street corners everywhere if it did not require the use of flying brooms. Typical of the heavenly whimsy that keeps the Potter books so freewheeling is the catalog of souvenirs described here:

"There were luminous rosettes -- green for Ireland, red for Bulgaria -- which were squealing the names of the players, pointed green hats bedecked with dancing shamrocks, Bulgarian scarves adorned with lions that really roared, flags from both countries that played their national anthems as they were waved; there were tiny models of Firebolts that really flew, and collectible figures of famous players, which strolled across the palm of your hand, preening themselves." Also Omnioculars, binoculars capable of instant replay.

The teams are cheered on, respectively, by tiny leprechauns in the sky who align themselves in the shape of rude hand gestures when Ireland is losing, and gorgeous blonde female creatures called veela who develop the beaks and scaly wings of birds of prey when they get angry. "And that, boys," the Weasley father announces, "is why you should never go for looks alone!"

Then it's on to Hogwarts, still the home of Ms. Rowling's most irresistible characters and creations, like the Sorting Hat in charge of assigning new students to each of the school's four houses; to do this, the Hat perches on a stool, opens its brim and begins singing. "Sings a different one every year," remarks Ron, the Weasley who is one of Harry's best friends.

"It's got to be a pretty boring life, hasn't it, being a hat? I suppose it spends all year making up the next one." Much of "Goblet of Fire" is devoted to enriching the already witty, madly colorful portrait of Hogwarts life that Ms. Rowling has continued to tweak merrily and has not allowed to grow tired.

Eventually it develops that Hogwarts will be engaged in a special contest this year: a Triwizard Tournament that involves two other schools, the stern Durmstrang and the rather more glamorous Beauxbatons. Each will be represented by one participant in an event that spans the entire school year and that, needless to say, involves Harry on the Hogwarts side, complete with tabloid press coverage that satirizes the treatment of Britain's schoolboy princes. Ms. Rowling would be profoundly disappointing her readers if she did not link the contest, with small clues salted all through the narrative, to the complex and ominous machinations of You-Know-Who and friends.

The series's first book served primarily as an introduction, though its intrigue involved a three-headed monster guarding the Sorcerer's Stone of the title. Then, in the weaker "Harry Potter and the Chamber of Secrets," Ms. Rowling drifted into a ghastly special effects denouement, replete with giant spiders, that provided the books' most unappetizing scenario. With "Harry Potter and the Prisoner of Azkaban," she arrived at a more trickily convoluted finale, to the point where you might have fried an egg on the forehead of anyone trying to sort out the book's climactic moves.

This time she achieves her most lucid, well-plotted and exciting conclusion, complete with a spectacular wand-on-wand confrontation to recall Luke, Darth and their light sabers, enhanced by the identity-twisting tricks in which Ms. Rowling specializes. The book ends on a mournful note with the loss of one character, and with ominous, cliff-hanging hints of a next installment. Two things seem certain: it will involve giants and be awaited with justifiably bated breath.