We’ve all had daydreams about being a superhero, but how many of us have actually pursued that goal? I quote Dave Lizewski, the protagonist and narrator of “Kick-Ass”: “Like every serial killer already knows, eventually fantasizing just doesn’t do it anymore.”

Dave, a relatively normal, semi-geeky New York teenager played by Aaron Johnson, decides to do something about it. “Kick-Ass” is an account of his adventures, and it’s a bawdy, bone-crunching, often hilarious, somewhat uneven action comedy brimming with outrageously vulgar jokes. Imagine if Judd Apatow and Robert Rodriguez teamed up to make a Spider-Man movie.

Like many American teenage boys, Dave and his friends, Marty (Clark Duke) and Todd (Evan Peters), read comic books and talk about girls. But Dave wonders: Why has no one ever tried to re-create himself as a superhero? Batman proved that you don’t need supernatural powers to fight crime. All you really need is a mask, a name, some fighting skills, and maybe some of Bruce Wayne’s gadget-development funding. Lacking the money and most of the skills, Dave settles for a home-made costume and a name: Kick-Ass. Apparently he has forgotten the first rule of P.R.: Don’t call your product something that some news outlets won’t say on the air.

Vigilantism proves harder than Dave expected, but eventually Kick-Ass becomes a sensation, with video of his mildly impressive exploits burning up YouTube. The girl he’s in love with, Katie (Lyndsy Fonseca), is among those smitten with the masked avenger, but of course she doesn’t know it’s him. She’s only interested in Dave as a friend, and only because she thinks he’s gay — one of several odd, ill-fitting threads in the film. (The way it turns out is even more perplexing. Is the movie making fun of hastily resolved plot devices, or is it merely employing one?)

As Kick-Ass goes about helping people who have brought their plights to his attention through his MySpace page, he encounters other amateur heroes who are taking the whole thing a lot more seriously than he is, namely a 12-year-old assassin called Hit-Girl (Chloe Grace Moretz) and her father, Big Daddy (Nicolas Cage). These two are more of the Bruce Wayne stripe, right down to the vast financial resources and the personal vendetta. They are unlike Batman, however, in that the Caped Crusader is adamant about not killing anyone, while Hit-Girl and Big Daddy have no such compunction. Through them, Kick-Ass gets involved in a caper surrounding millionaire crime boss Frank D’Amico (Mark Strong) and his son, Chris (Christopher Mintz-Plasse), a schoolmate of Dave’s.

Director Matthew Vaughn (“Layer Cake,” “Stardust”), who wrote the screenplay with his “Stardust” collaborator Jane Goldman, seems to have faithfully followed the vision of Mark Millar and John Romita Jr.’s comic book. No attempt has been made to soften the violent, rowdy humor for the big screen, nor to sidestep the fact that one of the most lethal and foul-mouthed characters is a preteen girl. On the contrary, Vaughn has embraced these elements — not in a “Look how naughty I am!” way, but in an “It’s that kind of story; take it or leave it” way. You don’t want to see taboos busted for the sake of comedy, don’t go.

Johnson, Moretz, and Cage are clearly having a good time in their roles (especially Cage), and the film does not lack for big laughs and enjoyably bloody action. It needs more footage of Kick-Ass making a name for himself, though — as it is, his fame seems to be based on one YouTube video. It’s also not always clear what the movie’s attitude is. Sometimes it’s an homage to comic books that puts them in a real-world context; sometimes it feels more like a parody; sometimes the real world is abandoned altogether. Dave’s ability to withstand a lot of pain is referenced in just two lines of dialogue, as if someone felt an explanation was needed but didn’t want to spoil the image of Dave as an ordinary kid with no powers. (He does have powers; we just don’t want to dwell on it.) This mix of fantasy and reality often seems unintentional — but it’s not enough to detract from what is otherwise a raucous crowd-pleaser, one that indeed kicks sufficient ass.

B (1 hr., 44 min.; R, abundant harsh profanity, abundant graphic violence, some sexual dialogue, brief partial nudity.)