ROBERT DE NIRO AS CAPTAIN SHAKESPEARE

“Stardust,” based on a Neil Gaiman novel, opens with the intonations of out actor Ian McKellen telling viewers about stars looking down from the sky at humans and humans looking up at the sky at stars. As the narrator, he also describes the city of Wall and its portal to Stormhold, a magical world. And so begins this epic romantic fable about Tristran Thorne (Charlie Cox), a teenager on a quest through which he becomes a man.

Queer viewers may identify with Tristran’s story, as he is an outsider who does not fit in with the popular crowd. “People aren’t what they seem,” he is told, and in his adventures, Tristran tangles with many shape-shifters — evil witches, power-hungry princes and even a tough pirate named Captain Shakespeare (Robert De Niro), who likes to dress up in drag and perform the “Can-Can.”

But as the narrator might indicate, this is all getting ahead of the story. Tristran’s tale begins when he declares his love for Victoria (Sienna Miller) and promises to bring back a falling star from the forbidden land of Stormhold for her. Unbeknownst to him, the king of Stormhold (Peter O’Toole) is looking for the heir to his throne and challenges surviving sons Primus (Jason Flemyng), Secondus (Rupert Everett) and Septimus (Mark Strong) to find the star — a stone that will turn back into a ruby in the hands of the true heir. Complicating matters, a trio of ugly old witches, led by Lamia (Michelle Pfeiffer), seek the star/stone to in the hope that it will give them eternal youth.

The jewel belongs to Yvaine (Claire Danes), the female embodiment of the star, and when Tristran meets her, it is loathe at first sight. She reluctantly accompanies him on his journey back home, which, of course, is fraught with deception — the pair encounter plenty of witchcraft — but also assistance from unlikely and sometimes untrustworthy sources.

“Stardust” does an incredible job of bringing Gaiman’s fantasy world to life, and the special effects, albeit computer-enhanced, help buildings materialize out of thin air and transform people into animals and back again. There may be a cute running gag involving the “Greek Chorus” appearance of the various dead princes — one with an ax in his head, another with blood dripping out of his slit neck and a third with his face crushed on one side from a fall — but these moments smack of computers working overtime. However fabulous the imagery is, one cannot help but wonder how much better, how much more inventive this film might have been if someone like Terry Gilliam had directed it.

The main problem with “Stardust” is that writer/director Matthew Vaughn (“Layer Cake”) fails to make it magical. The action lumbers from one set piece to the next, stopping almost dead in its tracks when Captain Shakespeare shows up. This sequence, set in the clouds, involves an overlong episode and a montage of Tristran learning to fence while Yvaine learns to dance. Meanwhile, Captain Shakespeare worries about his reputation being sullied should the secret of his cross-dressing be found out by his men or his enemies.

Alas, once the story gets back on track, there is little dramatic tension as the plots and characters eventually converge. A key scene where a witch in woman’s clothing dupes Tristran focuses less on the drama of the stone ending up in the wrong hands and more on the relationship developing between Tristran and Yvaine. Yet the emphasis is misplaced here, especially since the romance never sparks.

Another significant drawback is that Danes is badly miscast as Yvaine. Though radiant in her role — thanks to the help of the animated halos that surround her — nothing else about her work here is convincing, especially not her flimsy British accent. Thankfully, Cox fares much better as Tristran, and he has an engaging screen presence that makes him a hero to cheer for.

In support, Pfeiffer is wonderful in or out of her witch makeup and De Niro seems to enjoy sending up his tough-guy image, even if his sequence is the weakest.

“Stardust” could certainly enchant young viewers, who might respond favorably to the escapades on display, but overall this fantasy is just not that fantastic.