A previously unknown variety of three-toed lizard begins attacking children in Costa Rica. A sample carcass of the lizard is sent to a lab at Columbia University, where a lab technician, believing it is a dinosaur, calls the renowned paleontologist, Dr. Alan Grant. When Grant receives a fax of the lizard's skeleton, he is shocked to see that it is in fact a dinosaur. Before he can investigate any further, however, Grant and his research partner, Dr. Ellie Sattler, are flown to Isla Nublar, an island off Costa Rica, as consultants for InGen, a bioengineering firm.
John Hammond, the owner of InGen and an important financial supporter of Grant's fossil digs, has turned Isla Nublar into a zoo called Jurassic Park, which is stocked with dinosaurs that Dr. Wu has cloned by means of a breakthrough genetic engineering technology. Donald Gennaro, Hammond's lawyer, is concerned about the safety of the park. Gennaro brings Grant, Sattler, and the mathematician Ian Malcolm, who is also acting as a consultant for InGen, to tour the island in an attempt to determine whether the place is safe enough for visitors.
Malcolm is particularly convinced that the island is doomed, and makes repeated reference to a mathematical principle called chaos theory as he predicts disaster. Another visitor to the island is Dennis Nedry, the computer technician who designed the park's complicated computer network. Unbeknownst to Hammond and the others, the Biosyn Corporation, a rival bioengineering company, has enlisted Nedry to steal fifteen dinosaur embryos from the island for their own purposes.
Jurassic Park has been designed to run with only a minimal number of staff, with much of the park operating on automated, computer-driven technology and machinery. When Nedry shuts down the park's security system to steal the embryos, therefore, virtually all of the park's other systems start to malfunction. Nedry gets lost on his way to the docks—where he plans to deliver the embryos to a ship that is scheduled to meet him—and is killed by an escaped dilophosaur, a venom-spitting dinosaur. Nedry's death leaves John Arnold, the park's engineer, to try and unscramble the system lockdown Nedry instigated before he disappeared.
Meanwhile, the power outage has left Grant, Malcolm, the park publicist Ed Regis, and Hammond's two grandchildren, Tim and Lex, stuck in their electric guided-tour cars just outside the tyrannosaurus paddock. Looking through her binoculars, Lex notices that some small dinosaurs have stowed away on the supply ship that has just left the island.
The power outage has also disabled the electrified fences around the dinosaur paddocks. The t-rex breaks through the dormant fence, eating Regis and severely injuring Malcolm. Grant and the children are forced to flee out into the park on foot. The park's game warden, Robert Muldoon, joins with Gennaro to search for the others. They find Malcolm and bring him into the care of Sattler and the park's veterinarian, Dr. Harding.
Before too long, Arnold has the computer system running again and the power back up. Malcolm warns that, according to chaos theory, things will soon get much worse. Muldoon and Gennaro venture out again to find the t-rex. Meanwhile, Grant and the kids make their way back to the visitor center, narrowly escaping some pterodactyls and the t-rex again. Grant finds evidence proving that the dinosaurs have been breeding, even though the park scientists have claimed the dinosaurs were engineered to all be female.
By the time they get back to the lodge, the power has gone down again and the velociraptors—very intelligent and vicious small dinosaurs that hunt in packs—are loose on the island. The raptors kill Arnold while he tries to turn on the generator. They then surround the lodge, where Malcolm, Sattler, Harding, Wu, Muldoon, and Hammond are holed up. Grant is forced to leave Tim and Lex in the cafeteria while he goes to turn on the power. Wu is killed while helping Sattler distract the raptors from Grant. Grant finds Gennaro, who had disappeared, and turns on the generator. Meanwhile, a raptor stalks the kids in the cafeteria, but Tim locks it in the freezer.
Since everyone who knows how to use computer system is now dead, Tim, who is relatively computer-savvy, tries to reactivate the electric fences himself. He manages to do so just before the raptors are about to penetrate the electrified bars on the lodge skylight. Tim then calls the supply ship, which is about to dock in Costa Rica, and Gennaro commands the ship to turn around because of the velociraptors that have jumped aboard.
Grant, Gennaro, Sattler, and Muldoon venture out to find the velociraptor nest, hoping to determine how many raptors there are and whether any have gotten off the island. While walking outside the lodge, Hammond trips and falls down a ravine, where he is attacked and eaten by a group of small dinosaurs. Malcolm also dies from complications from his earlier injury. Finally, the Costa Rican National Guard show up, take everyone away, and blow up the island.
Grant, while waiting for the Costa Rican officials to release him, is approached by a man who tells him about some suspicious and unidentified lizards that have recently been seen traveling in packs through the jungle on mainland Costa Rica. The lizards have since fled deep into the jungle, and now no one knows where they are.