We knew it would be dumb. But we had no idea it would be so much dumb fun.

"Transformers," opening this week, is so jokey that it's practically a goof on the crummy TV cartoon it is based on. Blowing through the entire Dreamworks-Paramount explosives budget for the year and joked up and acted by Shia LaBeouf as if he's auditioning for the Royal Shakespeare Company, it's downright giddy for the first hour or so.

Then the stupid robots start talking and the whole enterprise crumples like a Cobalt in a compactor.

In the best "Independence Day" tradition, we're introduced to the characters — soldiers, computer geeks — and the peril facing the planet. There's a sandy assault on a U.S. base in the Middle East in which a helicopter transforms into a monster that plows through tanks, planes and Army Rangers like a bush hog in deep grass.

Pentagon security networks are compromised. And everything from police cars to boom boxes morphs into something menacing.

"This is as bad as it's ever goin' to get," drawls the Secretary of Defense (Jon Voight).

Which puts a serious crimp in the hopes of Sam Witwicky (LaBeouf), who only wants to buy his first car and get the attention of his school's Jennifer Connelly look-alike (Megan Fox).

But that magic first automobile is an aged Camaro that Sam doesn't choose. It chooses him. The rusted yellow muscle car has a need to protect Sam and help its fellow General Motors products find something called The Cube, which contains the One Spark (probably an AC Delco product).

All of this is zipped through in a giggling hour-plus of rapid-fire LaBeouf banter, sight gags (the alien cop car has "To Punish and Enslave" painted in place of "To Protect and Serve") and Chihuahua-in-a-cast jokes. The Camaro's AM radio helps Sam court the lovely Mikaela with tunes ranging from "Drive" (by The Cars) to "Sexual Healing," "I Feel Good" and "Baby Come Back."

Cameos by Bernie Mac (as a car salesman) and Anthony Anderson score. A comically threatening government agent (John Turturro) shows up. Assorted teen computer whizzes are introduced and all but forgotten.

Then the showdown begins.

"Transformers" hails from the Fecal Age of TV cartoons — dim-witted '80s junk solely aimed at selling the toys that were "the stars." Appropriately, the movie is basically a two-hour-and-20-minute ad for GM — Chevys, Pontiacs, Caddy Escalades.

Director Michael Bay, returning to his pre-"The Island" form of entertaining junk food movies, keeps things loose and fast for much of the film — so fast that the robot-on-robot fights are metallic blurs. He does a fair job of hiding the most sophomoric stuff (robots copping "street" attitudes, imitating Clint Eastwood or John Wayne, and adapting hip-hop speak). Fans who grew up with the TV show will recognize some of the voices of the gear-grinders — Optimus Prime, the Autobots (good robots) leader, is voiced by the TV Optimus.

Robots will throw down. Robots will die. Then Optimus goes all metal-messianic on us, marveling at the human race's bravery, and a lot of that fun is undone.

Thank Hasbro for Shia LaBeouf. He hurls himself into this as if his very career depended on it. His glib interplay with his dad (Kevin Dunn), offhanded putdowns and reactions to the lunacy going on around him are out there and on the money. Best of all, when he's running for his life, he's running for his life.

It's a summer popcorn movie, so you take the good with the ready-for-recall. It doesn't have "Die Hard's" body count, "Spider-Man's" gravitas or "Pirates' " obscene length.

But if you grew up with the 'toon, it'll be a fun ride down memory lane — in a Chevrolet.