In the opening moments of "Shoot 'Em Up," a homeless man sitting at a bus stop eating a carrot sees a woman who is clearly about to give birth being chased into an alley by a gunman. He feels obligated to help her, so he kills the gunman. With the carrot. Then the movie gets weird.

After the failure of similarly outlandish action flicks like "Crank," "Smokin' Aces," and "Running Scared," I'm surprised that a studio has invested in a theatrical release for "Shoot 'Em Up," especially considering the previous films by its writer/director, Michael Davis, have gone straight to video.

But I'm glad New Line is crazy enough to release it. Its over-the-top action is riotously entertaining, not to mention relentless: This movie barely stops to catch its breath, running from one ridiculous stunt sequence to the next. What's more, the film mocks the excesses of ... well, of movies like this. It's a congenial indictment of American gun culture and our fondness for loud, violent movies, and it's the most enjoyable indictment I've ever had the pleasure to receive.

The homeless man is called Smith, and he's played by Clive Owen, the dashing and rugged British actor who could have been James Bond. Smith squats in an abandoned building, where he grows his own carrots. Helps the eyesight, he says. There are several times in the movie where the camera focuses on his eye and then shows us what his keen senses are detecting that would not be visible to the average, non-carrot-consuming person. Also, as noted, a carrot can be useful as a weapon.

Smith has had training as a killer, obviously; one wonders if he was Jason Bourne's classmate. He has a long list of things he hates, including rich jerks who don't signal when they change lanes in their BMWs, and guys in their 40s who wear ponytails.

He also doesn't have much patience for people who try to kill pregnant women or their babies. After delivering the boy but failing to save the mother's life, Smith goes to DQ (Monica Bellucci), a prostitute who happens to specialize in breast-feeding, for help. Together they flee the henchmen of Mr. Hertz (Paul Giamatti), who wants this baby dead for reasons unknown.

Why would someone want to kill a baby? It all leads to an elaborate -- and knowingly absurd -- plot relating to gun control and a bone-marrow transplant and the presidential election. Really!

Walking the line between parody and plain old goofiness, Davis fills the characters' mouths with jokes and puns. The dialogue is often so ridiculous that even the characters don't buy it.

"I can't go to the police," Smith tells DQ. "I'm the Unabomber."

"They caught the Unabomber," she says.

"That's what they think."

Whenever Smith kills a bad guy (which is often) he cannot resist delivering a deadpan joke afterward, based on the manner in which the person died. It will remind you of the corny jokes uttered by Stallone and Schwarzenegger and Bond, or at least of the "Simpsons" parodies of movies like that.

Clive Owen, generally a serious actor, gives nice faux-gravity to his implausible character, while Paul Giamatti revels gleefully in playing a screamy, irrational bad guy, one whose wife is constantly bothering him on his cell phone. His phone plays "Ride of the Valkyries" whenever she calls, which makes you sing "Kill the wabbit, kill the wabbit," which makes perfect sense given Smith's fondness for carrots and his impossible resourcefulness. Like Bugs Bunny, he can pull a necessary prop out of nowhere -- including, at one point, an animatronic fake baby.

Within the genre of baffling, what-the-hell-is-this? movies, "Shoot 'Em Up" is a fine specimen. It's full of gunfire without being vicious (rarely do we actually see any blood), and it retains a certain good-naturedness even when guys' ponytails are being shot off. And don't worry, the baby makes it out fine.

Grade: B+

Rated R, a lot of harsh profanity, some nudity and sexuality, abundant shooting violence, though there are only a few brief glimpses of blood and gore

1 hr., 29 min.