Pirates of the Caribbean: At World's End (12A)

What a load of old cannonballs. The only thing that holds good in this, the concluding part of the Pirates trilogy, is the law of diminishing returns.

The voyage that began in the summer of 2003 with The Curse of the Black Pearl was driven along by a certain breeziness and sense of novelty. By the time of its sequel, Dead Man's Chest in 2006, that freshness had gone from its sails, yet it stayed buoyant on the raffish charm of Johnny Depp's performance as Captain Jack Sparrow. A year on, the wind has dropped altogether: the old tub At World's End thrashes about wildly but can't make any headway. It's a finale that will surely disappoint everyone but Jerry Bruckheimer and his shipmates, who will be cackling all the way to the box-office as they did with last year's record-breaking sequel.

One unfortunate consequence of these tipping points is their habit of retrospectively tarnishing what went before. With a few notable exceptions - The Godfather: Part II and Alien 3 come to mind - sequels tendto make you wonder whether the film that launched them was really any good in the first place. Casting back four years to the first Pirates movie, I suppose there must have been a fair cargo of silliness on board; it was, after all, based on a Disney theme-park attraction. Yet I don't recall anything like the tiresome to-ing and fro-ing that bedevils the rhythm of the present film. It reintroduces many of the original cast and draws on the same compound of cartoon swashbuckling and featherweight campery that has become the series' hallmark. But now, instead of one good story, the scriptwriters Ted Elliott and Terry Rossio contrive to lose us in a fog of competing plot-lines that always seem to involve someone being taken hostage, or being betrayed, or else requiring rescue of some kind.

It all begins in Singapore, where Captain Barbossa (Geoffrey Rush) and his reluctant allies Elizabeth (Keira Knightley) and Will (Orlando Bloom) have sought out a Chinese pirate captain (Chow Yun-Fat) who may furnish the charts that will direct them to world's end. This is apparently the location of Davy Jones' Locker, where their friend Jack Sparrow now languishes after his encounter with the Kraken at the end of the second movie.

But their enemy, the malignant Brit Lord Beckett (Tom Hollander), is in hot pursuit, having commandeered the ghost ship The Flying Dutchman, along with its squid-faced captain (Bill Nighy) and scaly crew. A flurry, a fight, a narrow escape, and it's off to the next location, while we wonder how securing the nine pieces of eight and the Nine Lords of the Brethren Court will help our heroes - and whether it's going to take as long as the second movie did.

Longer, as it turns out. This third instalment clocks in just shy of three hours, during which plots and counterplots are heaped on top of one another in such dizzying profusion that we hardly know (or care) what we are meant to be feeling. Allowed too much time to fill, the movie becomes complicated without being interesting, and frantic without being exciting.

On top of the high-seas shenanigans there is the maundering romance between Will and Elizabeth (with Jack as cuckoo in the nest); Will's unresolved relationship with his dad, Bootstrap Bill (Stellan Skarsgard); the possible involvement of Elizabeth's former suitor (Jack Davenport) in the murder of her father (Jonathan Pryce); the metamorphosis of Tia Dalma (Naomie Harris) into the sea goddess Calypso and, bringing up the rear, the brackish, snaggle-toothed pirate chorus of Mackenzie Crook et al.

On, interminably on, it goes, and in the absence of a story to beguile us, we look for a performance that might do the job. In the first two films it was Depp's Jack Sparrow who held the centre, the wild card whose preening dandyism hit the spot because he knew how absurd it really was, and knew that we knew. We first glimpse him here hallucinating within Davy Jones' Locker, captaining a whole crew of Sparrows and dragging his pirate ship over desert wastes.

Alas, this multiplicity of Sparrows could almost be a metaphor to express what's wrong with the film: the fallacy that if you keep adding to something you make it better. Instead, you dilute it, devalue it. Depp mugs and rolls his eyes, as before, but we wait in vain for him to speak a memorable line. His one good sight gag involves his rivalry with Barbossa over the length of their telescopes, a quasi-sexual one-upmanship that goes to the heart of his endearingly juvenile nature. But it's not much to show for three hours' attention.

Even Jack's much-anticipated reunion with his old dad is a squib. It was clever of them to hire a certain old rock guitarist for the role, not so clever to forgo writing the pair of them a proper scene together. Mind you, when the screenwriters do rouse themselves for a major scene, one hears the timbers begin to groan; Knightley's calling her shipmates to arms is plainly intended to be her big Agincourt moment, but they haven't mustered the words, and she hasn't the voice for it. While the director Gore Verbinski tries to invest the climactic battles - yes, there are a few - with some vigour and intensity, it seems that most of his time has been spent marshalling the CGI.

Will any of these failings confound the film's chances of blowing another hole through box-office records and beating last summer's monster success? Probably not. All the same, it will not leave anybody begging for more. They've picked these crossbones clean.

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