The main joke in "Teenage Mutant Ninja Turtles" is the very existence of its four heroes. The whole idea is intended to provoke a splutter of laughter: Superman, they ain't! After you get beyond that, though, what then?

For some fans, that's plenty. Launched in 1983 as a homegrown comic series by Peter Laird and Kevin Eastman, the Turtles eventually became part of the superhero genre they initially spoofed. By the 1990s they'd become a successful multimedia "property" -- toys, video games, the obligatory badly animated television series, a string of feature films. What goes around comes around: The fun-loving reptiles have lately been pushed aside by Batman and his fellow brooders.

Producer Michael Bay is hoping to do for the Turtles what he did for Transformers" (another dopey but popular franchise from the '80s) and aesthetically, he succeeds. The Turtles are just as hollow and charmless as Optimus Prime or Bumblebee, and they also co-star with Megan Fox, here doing a rather credible job as the intrepid reporter April O'Neil. As with "Transformers," what makes this movie tolerable is plenty of slick action sequences (the director is Jonathan Liebesman) and a decent human support cast (Will Arnett, East Meadow's William Fichtner, a brief Whoopi Goldberg).

The Turtles, named for Renaissance figures, used to be surfer-skater types, but here they've been slightly updated. Now we have tech-geek Donatello

(Jeremy Howard), loyal Leonardo (Pete Ploszek, oddly dubbed by Johnny Knoxville), rageaholic Raphael (Alan Ritchson) and hip-hopper Michaelangelo (Noel Fisher). Their sensei is a rat, Splinter (Danny Woodburn with Tony Shalhoub's voice), and their nemesis a chrome samurai named Shredder (Tohoru Masamune). The plot involves a chemical attack and an antidote.

Are the Turtles really any more preposterous than, say, the Wolverine (who also fought a chrome samurai recently)? If you think so, then you're probably already a fan. The rest of us probably won't quite get the joke.

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PLOT The half-shelled superheroes return.

RATING PG-13

CAST Megan Fox, Will Arnett, William Fichtner

LENGTH 1:41

BOTTOM LINE Rougher and slightly funnier than the 1990 original, but still harmless junk at best.