"Dark and difficult times lie ahead, Harry," our man Dumbledore intones at the end of Harry Potter and the Goblet of Fire, and as we've come to expect from men with beards as long as his, he's not wrong. There's trouble a-brewin', all right, but it ain't in the Potterverse: it's out here, where we legion of Potter faithful continue to jam multiple multiplexes full to bursting every time a new Harry Potter movie comes out. The problem is only that the Potter film franchise's handwriting is becoming more and more automatic with each successive installment, while author/empress J.K. Rowling's scribblings are becoming less and less automatable. Or to put it more bluntly: these stories are getting harder to tell, and the filmmakers would be wise to stop treating them like they're getting easier.

After a zesty (if rushed) take on Prisoner of Azkaban, itself built on detailed (if uninspired) takes on Chamber of Secrets and Philosopher's Stone, we've come to the fourth book in Rowling's hyper-successful fantasy saga, and gee whiz if the inherent complexities of the source novel don't just skyrocket clear past the limitations of the film franchise. Goblet is Rowling's most supple and emotional book of the series thus far, a delicate hinge-point for a 7-part series which both redirects everything that's happened before, and establishes everything that will come after. And unfortunately, "business as usual" in Harry Potter filmmaking just doesn't cut it this time out.

As a franchise, Potter is as venerable and secure as Bond, Star Wars, and Herbie the Love Bug all rolled into one. Director-proof and critic-proof, the Potter producers simply plunk the new elements into the mould every eighteen months, and pop out a new Potter souffle, almost without thinking about it. If J.K. Rowling had obliged by writing each novel with the same level of narrative complexity as the one before it, this would not be a problem.

She didn't, though, and Goblet doesn't just fall short of its novelistic source; I fear it might not even work as a movie on a basic, storytelling level. Have we reached the point where it's just assumed that everyone who goes to see these movies have already read the books, and the movies are therefore just colourful visual companion pieces with music and special effects?

I seriously doubt that Goblet can play for an uninformed audience at all; it hits all of the major story points with the same flair as the prior films (Harry's fight with the dragon is particularly luscious), but in between those points there's nothing. The movie is uniformly poor at telling its own tale; there are gigantic leaps of logic, horrible gaps in the plot, and a general unwillingness to explain its MacGuffin beyond a couple of shots of David Tennant looking creepy. (He's really creepy.)

The story revolves around a tournament at Hogwarts for the Tri-Wizard Cup, in which three young students are put through their wizardly paces by increasingly brutal tasks. No, four young students: naturally, Harry Potter (played to the nines once again by Daniel Radcliffe, who has buffed up, trimmed down, and generally hardened himself this time out) is unwillingly thrust into the competition. This earns him both the ire of his best friend Ron (Rupert Grint, whose stable of three facial expressions does not serve him well here) and the support of his other best friend Hermione (Emma Watson, dominant as ever but somewhat underutilized), along with blushing looks from scores of Hogwarts girls... but unfortunately not the girl, Cho Chang (Katie Leung, gorgeous as hell with a strange Irish accent). Even when you're a wizard, dating just sucks.

In this latter regard, Goblet soars past its predecessors, and for about fifteen minutes is the most accessible Potter film so far by a country mile. When the students are informed that they will have to attend the Yule Ball, and post-adolescent dating hysteria breaks out at Hogwarts, Goblet is nothing short of sublime. Everything - from Ron and Harry nervously comparing their threads before the dance, to a broken, Cinderellafied Hermione wilting on the steps of the Great Hall afterwards, tugging off her shoes in despair - is so perfect a portrait of being a 14-year-old dealing with his first school dance, that it actually inspired full-throated cheers from the audience.

When confronting the rising sexual maturity of its characters, Goblet clicks. There's a rather unabashedly naughty scene where Harry goes skinny-dipping to uncover the secrets of his Clue Egg (long story), only to discover that pervy spook Moaning Myrtle is gunning for a glimpse of his... well, guns. Ron and Harry watch in horror as everyone (even Fred/George Wesley in a hysterical wordless courtship of Angelina Johnson) pairs off with the opposite sex before our heroes can suck up the courage to ask someone to the dance. Neville (Matthew Lewis, shockingly gangly) grows balls the size of Church bells and has the date of his life with Ron's kid sister. And Harry's strangulated efforts at wooing Cho Chang match nicely with Hermione's awkward attempts at describing the "physical" charms of brutish suitor Viktor Krum: girls really do mature faster than boys, though talking about it with your friends can be hard. The kids are growing up, all right, and it is only here that Goblet actually paces its source and offers up something really fun and memorable.

The latest director, Mike Newell, has little interest in tampering with the formula. He delivers the task sequences with workmanlike skill, from taking Harry's fight with the Horntail up and out of the arena and into the open air, to finding a structure for the underwater adventure that both makes sense and looks good. He has more than his share of fun in bringing us the book's characters; Cedric Diggory literally falls out of the sky when we first meet him, and Malfoy is inexplicably up a tree in his notable effort at taunting Harry. Michael Gambon's Dumbledore improves measuredly over the last time around, but he's still a troublingly affable take on a necessarily mystical man; the producers would do well to consider recasting before the badass battles to come. Rita Skeeter's subplot is entirely unnecessary and shouldn't be taking time away from more important story elements, but Miranda Richardson's performance is so good that she deserves her own spin-off TV series. And when Brendan Gleeson stumps onto the screen, the whole movie nearly transcends, because Gleeson is such an apallingly good choice to play Moody that he could almost be an artistic statement all by himself. It's a credit to both the actor and the director that Moody's meaty, 8-minute explanation of the Unforigveable Curses is actually one of the best scenes in the film; it's not easy to turn laboured exposition into a gem like this.

The writing's on the wall, though; you can't take a 700-page book and winnow it down to a movie the same length as the one you made out of a 300-page book. Goblet of Fire feels like Coles Notes written by someone other than Cole. Screenwriter Steve Kloves' script races through the required plot points while never noticing the underlying truth: Goblet is the world's biggest gimme, because it's all written around a red herring. It's one of those situations where the story isn't the story; the Tri-Wizard Tournament, for all its razzle-dazzle, is window-dressing to the main event. Kloves seems to have missed this point; he skips the early (and vital) business with the Dark Mark at the Quidditch World Cup almost entirely, settling instead for an uneasy run-and-jump story starring Harry, but lacking the self-doubt and angst that made the book Harry's story.

That this leads to the other key beat from the book, where Lord Voldemort returns to the earth with a little help from Harry, seems almost accidental. To my knowledge, the film never even bothers to explain why the bad guys set up the Tri-Wizard Tournament in the first place, a sure sign that the filmmakers never twigged to the fact that Rowling's book makes a merry sport of us between its vitally important first hundred pages, and its equally important last hundred pages. As for Voldemort himself, it's a truly audacious take on the character from Ralph Fiennes, but all the mood lighting and CG facial reconstruction in the world can't quite make You Know Who as scary as he was when he was just an unnameable idea. (He's far more loathesome as a ferociously deformed skeletal baby, for about two bristling moments of the film.) Giving form to the Dark Lord makes him just one more fantasy heavy in black robes.

"Dark and difficult times lie ahead," indeed, but you'd never know it from the end of this picture; rather than bracing himself for the coming war, Harry merely seems content to quip with Ron and Hermione. It's the first time Radcliffe's performance begins to fail the material, because at the end of Goblet, a seismic shift should have taken place in the way this young wizard looks at the world around him. It hasn't, and he doesn't, and it's difficult to see how young Harry can get through the next thing, and the thing after that, and the thing after that, without having learned the lessons of death and sacrifice that Rowling so carefully taught him in the fourth book that bears his name. I worry about him. Harry is getting by just fine against the Dark Wizards, but can he survive his own film franchise?