Since the vast majority of the audience for "Spy Kid 3D: Game Over" has probably never seen a 3D movie with cheap, old-fashioned blue-and-red-lensed cardboard glasses, here's a three-point primer for proper enjoyment of any flick in this format:

1) Sit toward the middle of the theater. Because of the twin-image nature of 3D projection, the more off-center you are from the screen, the more you'll see eye-straining "ghosting" of images through your glasses instead of proper depth of field.

2) The left lens (red) always seems uncomfortably darker than the right (blue). Get used to it.

3) 3D movies are never as good as they are cool because the gimmick is the driving force, with plot and character development getting little more than lip service.

This last point is doubly true of "Kids 3" since most of it takes place inside a video game. As the film begins, Juni Cortez (Daryl Sabara) -- the franchise's saucer-eyed, insecure, adolescent junior James Bond -- comes out of bitter retirement (between pictures he'd resigned from the kiddie spy agency in a huff for no explored reason) to rescue the mind of his sister Carmen (Alexa Vega), which has been trapped inside a virtual reality game.

Designed by a villain called The Toymaker (a hammy, under-rehearsed Sylvester Stallone) to capture the minds of children for unexplained evil purposes, the game is like a 21st century "Tron" in which Juni joins forces with a band of teenage beta testers from around the world (the game is a prototype) to surf in lava, bounce around on "pogo-toads," engage in giant robot fights and race monster-shopped rocket-cycles along dangerous high-rise tracks -- all in order to reach Level Four, where Carmen is being held in suspended animation.

The all-CGI, cartoon-inspired game F/X are undeniably cool in three dimensions, and writer-director Robert Rodriguez gets a whole lot of mileage out of the movie's visuals, thanks in part to young Sabara's convincing interaction with a world that was created in post-production (95 percent of the movie was shot on a green-screen stage). It's a pity his acting against other humans doesn't measure up.

But Rodriguez aims lamentably low in almost every other aspect of the movie, with slapdash scripting, a determination to ignore common-sense plot holes in almost every scene, and a bad habit of laying it on thick with the sentimental candy-store philosophy.

"You are a superhero -- out there in the real world, to me!" Juni bleats to his wheelchair-bound grandfather (Ricardo Montalban) who has joined him in the game, gained virtual mobility and become reluctant to leave.

Montalban's arrival in the game is a good example of Rodriguez's who-cares-it's-just-a-kids'-movie attitude toward plugging the picture's flagrant story fissures. When he first appears, Grandpa Cortez is surprised to find himself in the game, even though in the real world someone would have had to find him, bring him to spy headquarters and hook him up to a console.

Such problems extend into every aspect of the story. Why does Juni have exactly 12 hours to find his sister? If The Toymaker is imprisoned in cyberspace -- as is explained by kiddie-spy HQ wonks played by Salma Hayek and Mike Judge -- why don't the good guys have more control over him? If the game is dangerous, why don't they stop it from being distributed?

The rules of the game are equally nonsensical. How does The Toymaker plan to enslave the minds of the world's children if players are returned to reality when they lose all their game lives? What does he plan to do with them once he's enslaved them anyway? And why do the beta testers think they're qualified to win the game's unnamed ultimate prize? "I'm here for the Great Prize beyond Level Five," one declares, "to save my family from poverty!" Oh, brother.

None of this means "Spy Kids 3D" isn't enjoyable enough to keep kids and parents mildly entertained. After all, "Spy Kids" and "Spy Kids 2" weren't exactly intelligent or coherent and they passed muster as matinee fodder. But without the deep field 3D effects serving as a distraction, a movie this unremittingly, unapologetically undemanding wouldn't stand up to even mild scrutiny.