Pirates of the Caribbean: Dead Man's Chest (12A) <!-- none onestar twostar threestar fourstar fivestar -->

Well, stab me vitals if it ain't another block-buster sequel about to splash down and capsize cinemagoers everywhere. The difference here is that some of us have actually been looking forward to the second instalment of Pirates of the Caribbean, so tickled were we by the first. The Curse of The Black Pearl wasn't exactly landmark cinema, but its lusty, headlong adventure was salted with the kind of self-mocking wit one barely expected of a movie whose creative origins lay in a Disney theme-park attraction. That it also sailed beneath the ensign of Jerry Bruckheimer - a producer whose quality control might best be represented by the skull and crossbones - only made it the more remarkable.

This second outing gathers together many of the original cast, and trades on the same rollicking mixture of cartoonish derring-do, supernatural spookery and daft comedy: the lively script (by Shrek co-writers Ted Elliott and Terry Rossio) ensures that the swash of a buckle is just as likely to be followed by the thud of a pratfall. If it has a fault then it's the one that bedevils so many event movies, namely an indefensible longwindedness. Dead Man's Chest clocks in only a few minutes longer than the first movie, but in the circuitous plotting and the elaboration of its setpieces it feels a less agile, more ponderous thing. And this, if you please, is only the middle panel of a trilogy (the third part was filmed simultaneously with the second), so even after two and a half hours of nautical shenanigans there's not a whiff of dramatic resolution.

Offsetting any potential slump, however, is the ludic - and occasionally ludicrous - presence of Johnny Depp as Captain Jack Sparrow, perhaps the most beloved screen pirate since Captain Pugwash. One might have thought it was a stunt he couldn't pull off twice, but Depp somehow seduces us all over again with a languid insouciance that seems to say: I can only just be bothered to entertain you. The eyes rimmed with kohl and the teeth glinting with gold are the standard piratical accoutrements; what lifts his performance is that mincing gait and mockney drawl, like a rockstar dandy from the 1970s suddenly required to wing it in a Restoration comedy. He carries off the funniest slapstick of the film when, captured by a tribe of flesh-eating natives, he desperately tries to avoid being kebabbed alongside a colourful array of jungle fruits.

The movie has reached this high point after some pretty sluggish convolutions of plotting that see Sparrow's friends from the first movie, Will (Orlando Bloom) and Elizabeth (Keira Knightley), torn asunder just when they were to be married. The agent of their misfortune is a cold-blooded colonial Brit (played with clipped hauteur by Tom Hollander) eager to get his hands on a magic compass which, by means of various other McGuffins including an ancient key and a chest, will grant him dominion over the Caribbean Sea trade. Meanwhile, Sparrow has eluded the Scylla of being barbecued only to run plum into the Charybdis of his old adversary Davy Jones, he of the locker fame, and captain of the ghost ship The Flying Dutchman. Jones, with a face that seems to have collided with an octopus, commands a slimy crew that is half-man, half-seafood, and stands as a mythical counterpart to Geoffrey Rush from the first movie; beneath the mound of prosthetics you may just recognise the rheumy eyes (though not the Scots brogue) of Bill Nighy. Inconveniently, Sparrow owes his immortal soul to Jones, and must now strike a bargain with him or else face an eternity of servitude - which is apparently even longer than this movie.

It's not that Dead Man's Chest is boring. The filmmakers indeed seem so paranoid about losing our attention that they keep nailing additional struts of plot on to the hull. These include the sorrowful reunion of Will and his long-lost dad, Bootstrap Bill (Stellan Skårsgard), the return of Jack Davenport as Elizabeth's former suitor, and the emergence of Naomie Harris as a swamp-dwelling sorceress with inky teeth - a part that would previously have been a shoo-in for Eartha Kitt. And let's not forget the sea monster that drags down unfortunate ships with its monstrous tentacles; there's no chance of forgetting it, actually, because the setpiece in which it wrecks a merchant vessel is repeated about half an hour later aboard the Black Pearl. It's a basic misapprehension of modern action-adventure movies that if you keep adding to something you make it more exciting. In fact, the law of diminishing returns must apply, as in the sequence where a mill wheel breaks off from a house and rolls down a hill while two men swordfight upon it. For thirty seconds it's quite exciting; after a couple of minutes you wonder why something improbable has also become interminable.

It might also be argued that the main romantic pairing is a damp squib (must be all that sea water). Orlando Bloom and Keira Knightley are a match for one another in prettiness, yet there's a distinct lack of sexiness about either of them. Their screen time together is minimal in any case, probably because the scriptwriters realise that nothing these young lovers do will be half as intriguing as the romance Jack Sparrow conducts with himself. I can't imagine anything this summer outdoing Depp's antics for style or silliness, and director Gore Verbinski seems to know that as long as his star is at the helm, then these Pirates will be ruling the box office high seas for some time to come.