If you part the roiling seas of The Chronicles of Narnia: The Voyage of the Dawn Treader, and look beyond the good vs. evil religious allegories ever present in the C.S. Lewis stories, you will discover the best thing about the present voyage—a rat and a brat.

For fans of the series, that means the long tail of Reepicheep is back with Simon Pegg giving excellent voice to the swashbuckling rodent whose rapier wit is as erudite as his swordsmanship is sharp. Though the Reepster will be facing greater foes, his biggest challenge is taming Eustace (Will Poulter), the obnoxious boy whose cousins Lucy (Georgie Henley) and Edmund (Skandar Keynes) have been forced to bunk with while the rest of the Pevensie clan is off doing more interesting things.

Alas, that is not enough to ensure smooth sailing for director Michael Apted’s Dawn Treader, which could do with more devils in the details, including the 3-D, yawn, ones. As it is, our heroes are forced to battle it out with—Green Mist. See, it doesn’t even seem scary when it’s capitalized. As is the case with most fairy tales, particularly ones as well loved and well read as Lewis’ Narnia series, you know going in how it’s all going to turn out, but the richness of the story keeps you coming back. Lewis knew enough to stir up much more than mist in Dawn Treader, which is probably why it will be his that we remember.

Still, on the big screen Narnia continues to be a lovely place for families to visit for a while with its all-knowing lion king Aslan, whose reassuring rumble is once again provided by Liam Neeson, always promising a better world. The introduction of Eustace, and a spirited performance by young Poulter, helps. Though the boy is an irritant, he’s also an avid journal keeper, with his entries providing some of the more biting narrative in a film in sore need of it. But as significantly, he’s the nonbeliever in the group—nearly as powerful a narrative force as evil, for Narnia, like Tinkerbell, thrives on the notion of childhood belief.

It’s still wartime England, 1943 now, and three years since the Pevensie siblings left Caspian (Ben Barnes) to settle into his kingly role. Susan (Anna Popplewell) is off in America with the unseen parents, while Peter (William Moseley) is busy with university exams, leaving Edmund and Lucy to finish growing up. He tries to join the fighting forces, she experiments with flirting, both get found out for the children they still are.

Most of their days, though, are spent quarreling with Eustace, and in the midst of an argument over the relative worth of fairy tales—you can guess where E stands on that one—they notice the ocean in a painting just above them seems to be spilling over the edge. Before you can say, “It’s all make believe, or CGI graphics,” they’re submerged in a watery portal to Narnia. It is one of the film’s best effects and far more difficult for the kids to navigate than the wardrobe of the first film, or the subway whoosh of the second. But what has always distinguished the Narnia films is a storybook beauty, ethereal in feel, a tradition continued here by venerable cinematographer Dante Spinotti, known for his high-gloss artistry, though usually on far grittier stuff such as 1999’s The Insider or 1997’s L.A. Confidential.

Apted, on the other hand, has long been involved in childhood dreams. You can trace it back to the extraordinary British documentary series that started with 1964’s Seven Up, looking at the lives of seven British kids. Apted began as a researcher but was soon directing the follow-ups over the years to see how things turned out. His best narrative films tend toward the uplifting sort as well, Gorillas in the Mist and Coal Miner’s Daughter among them, which would seem a good match for Narnia.

But the script he is working with for Dawn Treader, from Christopher Markus and Stephen McFreely, who wrote the first two Narnias, with Michael Petroni joining in for the third, ultimately fails. Oh, there are sword fights aplenty (as bloodless as ever), but instead of a real story, we are left clinging to individual moments. There are funny ones—the one-footed dwarfs (big foot, tiny bodies, lots of hopping); poignant ones—the magic dragon desperate to undo the spell; sort of scary ones—a ghastly sea serpent whose belly is the beast; and flat ones (too many to mention). What you won’t find is nearly enough tension as our heroes try to beat back the mist, and (there’s always an “and” in Narnia) recover the seven swords of the seven loyal lords who’ve somehow disappeared, along with the mystical, sprawling adventure Dawn Treader should be.