Back in the day, kids bought plastic “action figures” based on movie heroes. Now they watch movie heroes based on action figures. Like the Transformers saga, G.I. Joe: The Rise of Cobra is a film that owes its existence to a line of toys. So when you observe that supersoldier Scarlett (Rachel Nichols) has flame-red hair and gravity-defying cleavage, or that hero Duke (Channing Tatum) bears a conspicuous scar yet suffers multiple savage beatings without shedding a drop of blood, keep in mind that the filmmakers are merely reproducing the world of Hasbro with maximum verisimilitude. If you want your big-explosion movies to obey the laws of physics and logic, see The Hurt Locker.

Paramount executives chose not to prescreen G.I. Joe for critics, apparently because they were miffed by the “chasm” (as one rep put it) between the critical evisceration of the Transformers sequel and its record profits. They correctly foresaw that no pencil-head-geek critic was going to mistake G.I. Joe for an above-par popcorn movie like Iron Man, let alone a cinematic achievement.

The surprise is that it’s not that bad. Watching Transformers felt like being trapped on a three-hour car trip with a posse of preadolescents hopped up on Grand Theft Auto and Mountain Dew. But G.I. Joe is amiably, rather than aggressively, stupid. On the dumb fun meter, it occasionally reaches the level of director Stephen Sommers’ best previous effort, The Mummy.

It helps that Joe relies more on actors to deliver the goods and less on unwieldy mountains of pixels. (While computer effects are a major presence here, they don’t replace human characters.) As the villain, a weapons designer bent on world domination, Christopher Eccleston has all the beady-eyed intensity he brought to his role as the BBC’s ninth Doctor Who, plus a Scottish burr so thick an actual burr could be stuck in his throat. He’s aided by Joseph Gordon-Levitt (taking a break from quality films) as the mad doctor who unleashes swarms of “nano-mites” on unsuspecting cities, wearing a getup that makes him look like a cross between Marilyn Manson and Frank Booth using his inhaler in Blue Velvet.

If you were among the handful of people who saw Stop-Loss, you may remember Channing Tatum as a testy soldier returning from Iraq with Death Before Dishonor tattooed across his shoulderblades. Here he plays the square-jawed “true American hero” who never suffers combat fatigue, but don’t expect G.I. Joe to be steeped in red-blooded, red-state patriotism. It’s too absurd to be jingoistic, and the plot is less about protecting innocents from evildoers than about grabbing and hoarding toys.

Once Eccleston starts menacing the world with his nano-mite missiles — which look like snazzy lime-green canister vacuums you might buy at Target — his nemesis becomes General Hawk (Dennis Quaid), who runs the elite, international G.I. Joe Squad from an underground base in the Sahara. Special-ops guy Tatum talks his way into the unit based on his relationship with the Baroness (Sienna Miller), a leather-clad, superpowered evil henchwoman with whom he has a romantic history. Inside, he and his buddy Ripcord (Marlon Wayans) meet a bunch of other warriors with overdeveloped biceps and backstories.

While that may sound confusing, the story is never anything but a cinch to follow. The action sequences are cartoonish, but at least the costumes make it easy to keep track of who’s hitting or strangling or exploding whom. And the movie never forgets where its heart is. In a scene where the new “Joes” learn all about their cool suits and weapons, the soundtrack shifts into bow-chicka-bow mode. Next comes a training montage set to a cover of “Bang a Gong (Get It On).” For a few moments, the film ventures into satirical Team America territory with this “gun porn” that looks as sinister as an 8-year-old marching his action figures around the den.

For the most part, though, G.I. Joe lacks the wit of The Mummy. One of the few good lines goes to Miller, who is otherwise the cast’s weakest link. (She plays a turbo-bitch with the wanness of a Swiss-finishing-school girl.)

But the pace is good, the other actors are game, and the sets, costumes and effects are pure eye candy. If you’re up for a high-tech amusement-park ride without the whiplash — or just an excuse to spend two hours in an air-conditioned room — you could do worse.