The Tyrannosaurus was created when scientists took blood out of prehistoric dinosaurs and added present day animal DNA to it. The Tyrannosaurus was created with other dinosaurs and was meant to be the star attraction for the theme park. However, Dennis Nedry attempted to steal the dinosaur embryos and shut down the security system, allowing the dinosaurs to escape. The TYrannosaurus was amongst them. First she attacked paleontologist Alan Grant and two kids Tim and Lex. She crushed the car the kids were in, nearly killing them until Grant distracted the dinosaur with a flair. She also injured Malcolm and ate lawyer Gennaro. She then approached Alan and the children, and knocked them and the car off the bridge and into a tree and roared.
Later on, ranger Robert Muldoon and Alan's love interest Ellie Sattler searched for the missing people, stumbling upon the car and Gennaro's remains. Unfortunately, the Tyrannosaurus was also in the area, and came across them. The ranger and paleontologist, joined by Malcolm, drove away in the car while the Tyrannosaurus chased them. She was a fast runner, ramming the car once, but eventually ran out of breath and gave up.
The Tyrannosaurus targetted the other dinosaurs. She caught a fleeing Gallimimus in her jaws and shook it, killing it. She then fed on the dead dinosaur. At some point she spotted the velociraptors and the humans fighting and came on the scene. She snatched one raptor in her jaws and killed it. She was about to feed when the alpha-female, The Big One, attacked the Tyrannosaurus in anger. The Tyrannosaurus eventually gained the upper hand and broke the arptor's bones by biting her and threw her into a Tyrannosaurus skeleton. She then roared uin triumph and ate both raptors.