He immediately gets the shit kicked out of him, and nearly dies. As you would. But that doesn't stop him, and - metal head plates, pins in shattered bone structure, and all - he's back on the streets, and being largely ineffectual.
Which is when he comes across Hit Girl, a ten-year-old girl who is (to nick a phrase from the book) Polly Pocket crossed with Kill Bill's Uma Thurman.
The movie follows the first half of the comic book almost religiously: Goldman's script lifts sections of dialogue, and Vaughn's direction uses the panels as a reference as much as Snyder did for Watchmen. Things are shifted around a little in act II: we know Red Mist's identity from the start, and see the moments when he too decides to put on a mask. And the script's slightly smarter (don't have a MySpace page if you don't want people to track down the billing address with the ISP..., for example).
But it's the third act where Goldman leaves the book behind. Millar's original stays true to the characters - Kick-Ass, Big Daddy, Katie Deauxma - in ways that just don't fit with big, blockbuster endings, and where Millar rips the rug away, Goldman leaves it firmly in place.
That aside, it's a lot of fun. Comedy gangsters from the Guy Richie era, some gorgeous visual gags, and amazing fight scenes. Cage gets to have fun in an entirely invented sequence in a warehouse (added, I suspect, because Cage didn't want to get dressed up as Batman, but not actually get to be in any of the fights), but it's Chloe Moretz as Hit Girl who really stands out (and respect to fight arranger Rudolf Vrba and Vaughn, too, for giving the combat scenes such energy): the homage to The Matrix at the end is superb - as is the (slightly disturbing) choice of music.
Anyhoo: lots of fun, a hefty serving of gore (this isn't just comic-book violence), and I'd see it again in a heartbeat.