Grade: C-

Though not technically a sequel to Joe Johnston’s 1995 “Jumanji,” “Zathura” is also based on a book by Chris Van Allsburg about the amazing things that happen when a young boy unearths an old board game and makes the mistake of playing it. This time, though, what’s unleashed isn’t an orgy of jungle effects but an outer-space adventure, in which the house in which six-year old Danny (Jonah Bobo), his older brother Walter (Josh Hutcherson) and still-older sister Lisa (Kristen Stewart) happen to be ensconced is magically transported to the rings of Saturn, from which the brothers must “play” their way back to earth. Happily one throw of the dice brings them help in the shape of an astronaut (Dax Shepard) who proves somewhat intrusive but also knowledgeable and anxious to assist–for reasons that will be revealed at the end. Unfortunately, another leads to the arrival of a shipload of lizard-like aliens called Zorgons that prove distinctly destructive and hungry. Still, by learning, at the astronaut’s urging, to work together and support one another, the three siblings bond, overcome the rapacious beasts and make it back to terra firma.

There’s certainly nothing wrong with the ultimate message of the story–that brothers and sisters should set aside the squabbling and learn to appreciate each other’s company rather than taking one another for granted. And there are a few genuinely amusing moments, proving that director Jon Favreau, who worked wonders with Will Ferrell’s good-natured “Elf,” hasn’t entirely lost his touch. But most of “Zathura” is either shrill or flat. The bickering between Walter and Danny over the attention of their father (Tim Robbins, trying gamely to act the part of the concerned divorced dad in the first reel before disappearing for the rest of the picture), which continues for most of the running-time, gets extraordinarily tiresome, and Lisa’s nasty older-sister attitude, which alters only when the astronaut enters the scene, exacerbates the situation. And the young performers don’t prove ingratiating enough to make the kids a lovable trio: Bobo, the tyke from “Around the Bend,” is reasonably engaging, but Hutcherson is even more irksome than he was in “Little Manhattan,” and Stewart isn’t much more effervescent in her normal form than when the script turns her into a human popsicle (don’t ask). On the other hand, the oddly blase, slightly hazy stance that Shepard brings to the part of the spaceman never generates the charm it’s obviously intended to; at times, in fact, he seems to have wandered in from “The Hitchhiker’s Guide to the Galaxy”” (the awful movie version, that is). And the effects, though numerous and decently executed (Joe Bauer and Jon G. Belyou were the supervisors), are hardly enthralling–in fact, given the general improvement in the technology that has made such things cheaper and more easily achieved, the overall effect isn’t appreciably superior to what regularly appears on Saturday-morning TV nowadays. And it has to be said that the depiction of the Zorgons has nothing of the cutesy or comic about it; they’re fairly fearsome beasts that might frighten toddlers–as might a robot that proves less friendly than “The Iron Giant,” too. The profusion of explosions and firebursts that fill the picture might also be thought excessive–as certainly is John Debney’s score, which adds unearthly choruses to the instrumental mix to provide a sense of phony uplift far oftener than it should.

In a way the general homeliness of “Zathura” is a bit of a relief after the visual near-perfection of pictures like “A Series of Unfortunate Events” and “Charlie and the Chocolate Factory.” There’s a kind of clunky humility about it that’s kind of endearing for awhile. But by the time the picture approaches the hundred minute mark–overlong for a children’s movie–it’s become quite clear that “Zathura” has entirely too much to be humble about.