'For that you need a sheep," Penelope Cruz explains to Johnny Depp, as the pair huddle in a rough-and-tumble English pub. It is the early going of Pirates of the Caribbean: On Stranger Tides, the fourth installment in the Disney theme park ride-turned-3-D spectacle (and yes, you can pay extra doubloons for 3-D spectacles, and have a few swords come flying out of the screen, stabbin' at ya).

Depp, in his customary eye smudge, gold teeth, and buccaneer dreads is, of course, Jack Sparrow. And Cruz, it turns out, is an old flame named Angelica, busy recruiting her own crew of deck-swabbing scalawags for a seafaring mission. So what's this business about a sheep?

Oh, I get it, the Spanish siren, fresh to the Pirates series, is saying ship. "For that you need a ship."

Anchors aweigh.

An epic exercise in sequel-restructuring, with new characters coming in (notably Cruz and Ian McShane's Blackbeard) and old ones nowhere to be found (Orlando Bloom and Keira Knightley, bon voyage!), On Stranger Tides gets off to a rollicking start on 17th-century terra firma. King George has summoned Sparrow to his palace. Word has reached the monarch that a Spanish fleet is heading for the New World with a map of the Fountain of Youth, and the Brits must get there first. This royal confab doesn't go well for Sparrow, however, and so, after some tricky business in a London courtroom, some lockups and escapes from jail, and a chase through ye olde muddy streets, Jack finds himself in the company of the one-time convent sister (if she's to be believed) who became his lover and now seems to be working for Captain Blackbeard, or something like that. The Pirates franchise has never taken its back stories terribly seriously.

Directed by Rob Marshall (Chicago, Memoirs of a Geisha, Nine), who shows himself capable of orchestrating elaborate set pieces and overseeing squads of special-effects teams, On Stranger Tides is somewhat more buoyant than its two immediate forebears in this series. Depp sashays around as is his wont, dropping bon mots and bawdy puns - and getting a chance to chew the fat with his inspiration for the role, Keith Richards, once again. Geoffrey Rush is back as Barbossa, Jack's newly peg-legged rival, who is leading his own expedition to Ponce de Leon's legendary font.

And so the race is on: the Spaniards for one; Barbossa, a "privateer" flying the Union Jack, for two; and Sparrow and Angelica aboard Blackbeard's Queen Anne's Revenge, for three.

At a certain point, when landfall is made and a school of CG mermaids start splashing and diving (and, alas, showing vampire-like fangs), On Stranger Tides falls into the dark, effects-driven morass that made At World's End and Dead Man's Chest so torpid. (About those mermaids: Pre-Raphaelite supermodels, anyone?) There's little if anything here to keep us emotionally invested, and so the pile-it-on mutinies and sword fights and perilous dives off vertiginous cliffs exist just to wow us.

And frankly, the wow factor isn't that great. The quest for the Fountain of Youth - which requires the ritual deployment of a couple of silver chalices and a vial containing a mermaid's tear - turns out to be just a salty dog's McGuffin.EndText