The wealthy billionaire and dinosaur enthusiast John Hammond (Richard Attenborough), CEO of InGen has recently created Jurassic Park; a futuristic theme park populated with dinosaurs cloned from DNA taken from fossilized mosquitoes preserved in amber. The park is situated on Isla Nublar, an island 87 miles north-west of Costa Rica; for security purposes, the island is surrounded by five hundred miles of electric fence, and the dinosaurs have had their chromosomes altered to make them all female to prevent reproduction.
After an incident in the park, where a worker is fatally savaged by a Velociraptor, Hammond is advised by his lawyer Donald Gennaro (Martin Ferrero), to have two experts sign off on the island for the benefit of health and safety. To that end, Hammond invites paleontologist Alan Grant (Sam Neill), paleobotanist Ellie Sattler (Laura Dern) along with Dr. Ian Malcolm (Jeff Goldblum), a chaotician, along with Hammond's grandchildren—Timothy "Tim" Murphy (Joseph Mazzello) and Alexis "Lex" Murphy (Ariana Richards)—to Jurassic Park, in order to convince his lawyers the park is safe. The group is sent into the park as part of a safari experience to observe most of the animals. Hammond, meanwhile, observes his guests along with Head Technician Ray Arnold (Samuel L. Jackson) and his Head of Security, Robert Muldoon (Bob Peck).
However, it transpires that one of the computer programmers, Dennis Nedry (Wayne Knight), is in the employ of BioSyn, a corporate rival of InGen, who has been paid to steal fertilized dinosaur embryos for a substantial price. To allow the theft, Nedry infects the park's security measures with a computer virus to allow him access to the embryo storage, inadvertently deactivating the electrified fences containing the dinosaurs. The rest of the group, who have been stranded in the park due to the system shutdown, are attacked by the Tyrannosaurus escaping from its paddock; Gennaro is killed and Malcolm is badly injured. Grant survives and escapes with the children into the park. Nedry flees with his prize, but his jeep gets stuck in the Dilophosaurus paddock. The dinosaur blinds Nedry, then gets locked up along with him in his jeep and kills him, causing the embryos to be lost.
With the virus affecting the entire park's systems, Hammond recommends a total shutdown of the park's systems. Along with Sattler, Arnold, Muldoon and Malcolm, the group shuts down the park's system and retreats to the emergency bunker, from where Arnold heads to the maintenance compound to reboot the system. When he doesn't return, Muldoon and Sattler head for the compound; on the way, they pass the raptor paddock, and discover the raptors (whose fence wasn't shut off by Nedry but was affected by the park-wide shut-down) have escaped. At the same time, Grant and the children discover a nest full of hatched eggs; the dinosaurs are breeding. Grant theorizes this is due to using frog DNA to fill in gaps in the dinosaur gene sequence, as some frogs are able to change gender in a single-sex environment.
As Muldoon and Sattler proceed to the maintenance compound, Muldoon realizes they are being hunted by Velociraptors. The pair split up; Muldoon goes after the raptors, while Sattler heads for the maintenance shed. She manages to reactivate the park's systems (narrowly escaping a Velociraptor hiding inside the shed, which has killed Arnold); meanwhile, Muldoon is ambushed and killed by the Velociraptors. Grant and the kids head for the visitor's centre; he leaves the kids alone in the kitchen while he reunites with Sattler and the others. The kids find that two Velociraptors have entered the visitor centre, but they are able to evade the dinosaurs, reunite with Grant and Sattler and get the park's security systems working from the control room. Grant contacts Hammond and tells him to call the mainland for rescue, but the two raptors find the group and attack.
The group flees, only to be cornered in the entrance hall by the Velociraptors, who get ready to pounce. As all seems lost, the Tyrannosaurus breaks into the hall and seizes one of the raptors in its jaws, killing it. The second raptor charges the T-Rex and Sattler, Grant and the kids run for safety as the two dinosaurs fight; they are rescued by Malcolm and Hammond, who have fled the emergency bunker in a Jeep, where Grant tells Hammond he will not endorse Jurassic Park, a decision with which Hammond agrees. As they drive away, the Tyrannosaurus finishes off the Velociraptor and roars in triumph.
The group reaches the helipad, where they are evacuated from the island. As their helicopter flies across the ocean back to the mainland, Grant sees a flock of pelicans flying past, reminding him of the theory—that he advocates—that birds evolved from dinosaurs.