A documentary by Henry Alex Rubin and Dana Adam Shapiro. Rated 14A. Opens Friday, July 15, at the Fifth Avenue Cinemas

Suspension of disbelief is a critical element of fiction. When an audience is overly aware of trickery, then it cannot accept what it's seeing. This phenomenon is not normally an issue with documentary movies, which record real events. But in Murderball, the effect is reversed: the more you watch it, the more incredible it seems.

Murderball itself is a game; essentially, indoor rugby, played four a side. It's a loud, intense, full- contact sport, requiring agility, passing touch, and brute strength. The unbelievable part is that it is played in wheelchairs by quadriplegics.

How can people with this disability take part in such a violent activity? The technical answer, the movie explains, is that the term quadriplegic simply refers to impairment of all four limbs. Someone with a mobility rating of 0.5 points, like the late Christopher Reeve, would not have been able to participate. But someone with 3.5 points can really zoom. (Each team can only have eight points on the floor at one time.)

The emotional answer is that these guys are hard-core jocks. The attitude seems to be: Caution? What for? What's the worst that can happen-a broken neck? Hence, the furious melee and wild collisions that give the sport its name.

The directors, Henry Alex Rubin and Dana Adam Shapiro, don't stint on the game footage, which, to be fair, includes nifty finesse plays as well as those insane NASCAR-style spinning wipeouts. But they've also found us a tense story line and superb characters.

The movie follows the adventures of the Team USA quad-rugby squad as it prepares for, and competes in, the 2004 Paralympic Games in Athens, Greece. We focus first on Mark Zupan (3.0 points), the muscular, tattooed captain, as he leads his guys (no women made the national team) to victory in the 2002 world championship. Off the field, Zupan seems to be a bright, well-rounded guy (has hot girlfriend, works as a civil engineer), but on it he is a menace, and his Team USA is the colossus of the quad-rugby world.

During the run-up to the Paralympics, a real challenger appears: Team Canada. Its coach, Joe Soares, is a real piece of work. A legend in American murderball, the aging Soares reacted badly to being cut from the U.S. team-so badly that he took his knowledge of Team USA's tactics and personnel north, honing Team Canada into an instrument of revenge. He's a hell-fire, screamer coach, and a hard-ass father to boot.

And then there is Keith Cavill, a motocross racer newly recovering from spinal fractures. Life seems pathetic and dull to him until the day he attends a murderball clinic. Invited to take a quick spin in Zupan's awesome customized battle chair, Cavill can barely bring himself to leave it. A quad-rugby player is born, just in time to watch the showdown in Athens.

Which brings us to the amazing game itself: USA vs. Canada. These guys might not be able to walk, but can they ever fly! A stirring conclusion to one of the best sports documentaries ever made.