Industrialist John Hammond has created a theme park of cloned dinosaurs, Jurassic Park, on Isla Nublar, an island off the Costa Rican coast. After a dinosaur handler is killed by a Velociraptor, the park's investors, represented by lawyer Donald Gennaro, demand that experts visit the park and certify its safety. Gennaro invites mathematician and chaos-theorist Ian Malcolm, while Hammond invites paleontologist Dr. Alan Grant and paleobotanist Dr. Ellie Sattler. Upon arrival, the group is shocked to see a live Brachiosaurus.

At the park's visitor center, the group learns that the cloning was accomplished by extracting dinosaur DNA from prehistoric mosquitoes preserved in amber. DNA from frogs was used to fill in gaps in the genome of the dinosaurs, and to prevent breeding, all the dinosaurs were made female by omitting exposure to testosterone during a critical period in embryonic development. The group witnesses the hatching of a baby Velociraptor and visits the raptor enclosure. During lunch, the group debates the ethics of cloning and the creation of the park; Malcolm warns about the implications of genetic engineering and scoffs at the park's conceptualization, saying that it will inevitably break down.

The group is joined by Hammond's grandchildren, Lex and Tim Murphy, for a tour of the park, while Hammond oversees the tour from the control room. The tour does not go as planned, with most of the dinosaurs failing to appear and the group encountering a sick Triceratops; it is cut short as a tropical cyclone approaches Isla Nublar. Most of the park employees leave for the mainland on a boat while the visitors return to their electric tour vehicles, except Sattler, who stays behind with the park's veterinarian to study the Triceratops.

Jurassic Park's disgruntled lead computer programmer, Dennis Nedry, has been bribed by Dodgson, a man working for Hammond's corporate rival, to steal fertilized dinosaur embryos. Nedry deactivates the park's security system to gain access to the embryo storage room and stores the embryos inside a container disguised as a shaving cream can. Nedry's sabotage also cuts power to the tour vehicles, stranding them just as they near the park's Tyrannosaurus rex paddock. Most of the park's electric fences are deactivated as well, allowing the Tyrannosaurus to escape and attack the group. After the Tyrannosaurus overturns a tour vehicle, it injures Malcolm and devours Gennaro, while Grant, Lex and Tim escape. On his way to deliver the embryos to the island's docks, Nedry becomes lost in the rain, crashes his Jeep Wrangler, and is killed by a Dilophosaurus.

Sattler helps the game warden, Robert Muldoon, search for survivors; they recover an injured Malcolm, but are chased away by the returning Tyrannosaurus before they can find Grant, Tim, and Lex, who take shelter in a treetop and encounter a Brachiosaurus. Those three later discover the broken shells of dinosaur eggs, and Grant concludes that the dinosaurs have been breeding, which occurred because of their frog DNA—some West African frogs can change their sex in a single-sex environment, allowing the dinosaurs to do so as well.

Unable to decipher Nedry's code to reactivate the security system, Hammond and chief engineer Ray Arnold reboot the park's system. The group shuts down the park's grid and retreats to an emergency bunker, while Arnold heads to a maintenance shed to complete the rebooting process. When Arnold fails to return, Sattler and Muldoon head to the shed. They discover the shutdown has deactivated the remaining fences and released the Velociraptors. Muldoon distracts the raptors, while Sattler goes to turn the power back on, before being attacked by a raptor and discovering Arnold's severed arm. Meanwhile, Muldoon is caught off-guard and killed by the other two raptors.