The newest IMAX spectacle begins with a baby crawling along a beach and ends with a couple of guys on a go-cart track. In between it looks, sometimes with dizzying intensity, at the various methods we've developed to go faster, and then go faster and faster.

As a large-screen film, there are hardly any faults here. If it's excitement you want, there's plenty to go around. The viewer is plopped right in the middle of cars, boats, planes, and roller coasters zipping right along. Cameras stay with runners and swimmers and bicyclists, and at one point, follow a group of skydivers who are edging toward their jump door and then leaping out. Alas, the filmmakers chicken out at the last second; the camera doesn't follow them down, instead watching them flying away.

That's the only disappointment in a sequence that could have been so exciting as to have been nausea-inducing. (Then again, maybe that's for the better.)

Eventually, the 40-minute documentary settles down to focusing on just four people who are out to go as fast as humanly possible -- and then begin thinking immediately about besting their own records. What's really too bad is that once it goes that far, it doesn't go far enough. That's because it doesn't get deep enough inside the heads of its subjects.

Sprinter and five-time Olympic medalist Marion Jones is personable enough. She's seen on the track, both in practice and in competition, and her sleek machine of a body is a sight to behold. There's a tough-to-watch "agony of defeat" segment of an incident where her back went out on her a few years ago, but that's followed by her string of victories. From a scientific basis, the Jones segment is the most fascinating, because there's a well-presented explanation of how a runner's body works, featuring slow motion and stop-action techniques.

But even when Jones is shown joking around or seriously instructing the film's host, Tim Allen, in how to stand and how to run, the passion that obviously burns within her is never explained sufficiently.

The same goes for race car driver Lucas Luhr who, we're told, has a dream of winning the famous endurance test at Le Mans. He certainly gives it his all, practicing spin-outs in his Porsche (just in case one happens on the track), running through the woods to keep in shape, surviving a 10-hour race that consists of blistering heat, driving rain, day turning into night, and much fatigue. Why is he doing this? We don't find out.

At least some personal information comes out of mountain bike racer Marla Streb, who, at age 37, is convinced that she can beat any of her much-younger competitors in an upcoming race. There's some spectacular footage of Streb and her pals riding through Monument Valley in training, and one truly insane sequence in which she rides a bike down a bobsled course. But the business at hand -- why people want to go at such high speeds -- kind of takes a back seat to her argument of simply wanting to beat everyone else.

The inclusion of a fourth subject, Stephen Murkett, a race car designer, doesn't really fit with anyone else in the film. They're athletes, he's an engineer. Despite yet more magnificent scenery in aerial shots of his car being driven through the Canadian Rockies, there's no real point to his presence in the film.

Some interesting points are brought up. The script hints at the "game" of speed and suggests there are "rules" that have to be followed. But the rules are never fully explained. One subject that is explained well is the notion that these athletes often achieve their goals by letting their minds relax and actually slow down, in order to reach a state called "flow."

Allen is seen on-camera trying out some of the activities, and he lets loose from time to time with a one-liner or a goofy facial expression. But his selling point is some excellent, assured narration that mostly keeps track of what's going on.

The filmmakers chose to include an animated hummingbird, which flits by whenever a new segment is introduced. That's enjoyable the first couple of times. But it soon becomes distracting. The inclusion of a Mozart soundtrack, though, makes for a perfect match of sight and sound.

Top Speed is fun to watch and fun to listen to. But when it's over, one question lingers. Unfortunately, it's the one question that should have been answered at the get-go: Why do these people do what they do?