Insiders were pretty tight-lipped about the plot of the third Jurassic Park film, even up until the release of the movie. It could have been that they were instructed not to tell, or because they simply did not know (see the next section about the "problems" during production).
To summarize, however, JP3 takes place 4 years after the events in The Lost World. Near the start of the film, a young boy (Trevor Morgan) is parasailing near the island (now strictly off-limits, of course), when the line breaks and is stranded. His parents (Tea Leoni and William H. Macy) launch a rescue expedition, but look to Dr. Alan Grant (Sam Neill) for help. Without telling him the real reason they're returning to Isla Sorna, Grant agrees (after a little "persuading") to simply fly over the dinosaur santuary. Of course, nothing in Jurassic Park goes smoothly, and Grant learns they intend to land on the island. Despite Grant's protests, they head in, though the plane is quickly swatted to the ground by JP3's new dino, the spinosaurus.
Do you really need to know more than that? The film is jam-packed with action ("Action, action. It's very intense," says William H. Macy). "We've got dinosaurs you haven't seen before, and it's action pretty much from the beginning," says Sam Neill. "There's a lot of running in terror."