The film opens with a trio of young bluebirds harassing their youngest sibling, Buster (Blaze Berdahl). As Buster leaves his family, he meets an intelligent golf-playing orange Tyrannosaurus Rex named Rex (John Goodman). He explains to Buster that he was once a ravaging dinosaur, and proceeds to tell the story of how he came to become what he is today.

Millions of years in the past, Rex was a savage and frightening creature who terrorized smaller dinosaurs; he is seen giving chase to a Thescelosaurus. His rampage is interrupted when a futuristic aircraft arrives, and he is then greeted by a small green alien-like creature named Vorb (Jay Leno) who lures him into the craft and force feeds him "Brain Grain", a cereal that increases a creature's intelligence. Rex is given his name and introduced to other dinosaurs that have been fed Brain Grain: Dweeb the green Parasaurolophus (Charles Fleischer), Woog the blue Triceratops (Rene LeVant) and Elsa the lavender Pteranodon (Felicity Kendal), who develops an immediate crush on Rex. Later, they meet Captain Neweyes (Walter Cronkite), the inventor of the cereal and pilot of the aircraft, who then reveals his goal of allowing the children of the present time to see real dinosaurs, fulfilling their biggest wishes. He informs them that there are two people to watch for: Doctor Juliet Bleeb (Julia Child), a scientist from the Museum of Natural History who will aid the dinosaurs, and Professor Screweyes (Kenneth Mars), Neweyes' insane twin brother. He instructs them to find Bleeb and avoid Screweyes.

Neweyes drops the dinosaurs off at the Hudson River in 1993, where they meet a young raft-sailing boy named Louie (Joey Shea), who plans on running away to join the circus. Upon reaching New York City, Louie realizes that the citizens would panic if they saw live dinosaurs and, with the help of Elsa, flies over the city in search of a good hiding place. During the flight, Louie comes across a girl named Cecilia Nuthatch (Yeardley Smith), who was crying; she explains that her parents are hardly around (saying that her father is into business, and her mother is more of a socialite). Louie convinces Cecilia to abandon her home and introduces her to the dinosaurs (Louie and Cecilia then quickly develop an infatuation on each other).

When the dinosaurs explain their goal of getting to the Museum of Natural History, Louie decides to disguise them as floats in the Macy's Thanksgiving Day Parade. During the parade, Rex hears all the children wishing to see real dinosaurs, so he starts to sing (the movie's theme song) "Roll Back the Rock (To the Dawn of Time)". When the audience realizes that live dinosaurs are among them, they fly into a panic. The local authorities are called in to stop the dinosaurs, resulting in a slapstick-filled chase scene. As the dinosaurs lose the police, Louie and Cecilia venture to Central Park, where Professor Screweyes is running his "Eccentric Circus". They meet Professor Screweyes and cite their desire to join his circus. As they sign a contract in their own blood, the dinosaurs arrive and try to save them. Screweyes, upon meeting the dinosaurs, explains that he delights in scaring people and believes that the dinosaurs would make a great addition to his circus. He then reveals his very own "Brain Drain", pills that are the polar opposite of his brother's Brain Grain. He demonstrates the Brain Drain on the two children, devolving them into chimpanzees; however, he makes the dinosaurs a deal: if the dinosaurs consume the pills and join his circus, he'd destroy the contract and release Louie and Cecilia; the dinosaurs reluctantly and sadly accept the offer.

Believing that his friendship with the two would be lost, Rex, reverting Louie and Cecilia back to their human selves, tells them to remember him.

As Louie and Cecilia awake the next morning, they are greeted by a circus clown named Stubbs (Martin Short). Upon seeing the dinosaurs returned to their natural vicious states, Louie and Cecilia, with the help of Stubbs, plan to sneak into that night's show and save the dinosaurs. Professor Screweyes claims he can control the now wild Rex, which he succeeds in doing by hypnotizing him. Everyone watching the show gets frightened during the performance, making them even more scared that many run away, and after a crow turns on a highlight, causing the hypnotic effect to end, Rex becomes enraged, then he realizes he has been tricked and tries to attack Screweyes. However, Louie steps in and desperately tells the devolved Rex that killing Screweyes would be worse than what was happening at this moment. These impassioned pleas serve to return the dinosaurs to their friendly natures. Just then, Captain Neweyes arrives in his aircraft and congratulates Louie and Cecilia, who proceed to kiss in front of a whole crowd of people; simultaneously, Elsa tells Rex her true feelings towards him and they begin a relationship. After Stubbs arrives to resign from Professor Screweyes's employ, Captain Neweyes, Louie, Cecilia and the dinosaurs board the aircraft, leaving Screweyes to be devoured by the crows (that have been present throughout the movie).

The dinosaurs spend the rest of their days in the Museum of Natural History, allowing children to see live dinosaurs, fulfilling their wishes. Meanwhile, Louie and Cecilia reconcile with their respective parents, and the two become a couple. Rex returns Buster to his family before returning to the Museum of National History, humming the movie's theme song to himself.

• John Goodman as Rex The Tyrannosaurus Rex
• René LeVant as Woog The Triceratops
• Felicity Kendal as Elsa The Pterodactyl
• Charles Fleischer as Dweeb The Parasaurolophus
• Joey Shea as Louie
• Yeardley Smith as Cecilia Nuthatch
• Walter Cronkite as Captain Neweyes
• Jay Leno as Vorb
• Kenneth Mars as Professor Screweyes
• Martin Short as Stubbs the Clown
• Julia Child as Doctor Juliet Bleeb
• Blaze Berdahl as Buster
• Rhea Perlman as Buster's mother

To promote the film's release, a giant helium balloon of Rex the T. Rex was included in the real-life 1993 Macy's Thanksgiving Day Parade in New York. Unfortunately, as the parade moved through Columbus Circle, high winds caught the Rex balloon and caused it to lift over the nearby sidewalk. The head of the Rex balloon struck a protruding street light and popped, but the rest of the dinosaur's body remained inflated until the end of the parade. There is also a time in which during the parade the dinosaurs passed a movie theater where the famous science fiction film Jurassic Park is displayed on a sign above the theater.

There were also video game adaptations of the movie released for the SNES, Sega Genesis and Game Boy.

The movie got mixed to negative reviews. It scored a 27% on Rotten Tomatoes.

• A scene in which Professor Screweyes cages and chains the dinosaurs was fully animated, but ultimately cut from the movie on the grounds that it was too intense. Shots of it can still be seen in the original trailer.

• March 16, 1994 (VHS)
• May 26, 2009 (DVD)

The laserdisc release was presented in its original widescreen aspect ratio. When the film was released on DVD in some international countries, it was presented in the pan and scan format. However, the widescreen version of the film was once available at Hulu, but was removed. When the film was finally released for the first time ever on DVD in the United States and Canada on May 26, 2009, it was presented in anamorphic widescreen, being Universal/Amblin's first (and so far, only) animated film to be presented in widescreen on a Region 1 DVD (although international DVD releases of An American Tail: Fievel Goes West and Balto were presented in widescreen and some international DVD releases of An American Tail were presented in widescreen), and also Universal's second animated film from the 1990s to be presented in widescreen on a Region 1 DVD (the first being Jetsons: The Movie).

This soundtrack included the song "Roll Back the Rock (to the Dawn of Time)" by James Horner and Thomas Dolby.

Soundtrack album track listing

1. Main Title / Primeval Times – 4:14
2. Flying Forward in Time – 5:48
3. Welcome to New York – 2:26
4. First Wish, First Flight – 3:48
5. A Hint of Trouble / The 'Contract' – 1:49
6. Roll Back the Rock (to the Dawn of Time) (a Dinosaur Song): performed by John Goodman – 2:55
7. Grand Slam Demons – 2:05
8. Hot Pursuit – 3:18
9. Central Park – 1:21
10. Screweyes' Circus / Opening Act – 1:12
11. Circus – 2:29
12. Fright Radio / Rex's Sacrifice – 6:19
13. Grandparent Demon Parade – 7:39
14. The Kids Wake Up / A New Day – 2:57
15. The Transformation – 5:30
16. Special Visitors to the Museum of Natural History – 2:12
17. Roll Back the Rock (to the Dawn of Time): performed by Little Richard – 2:56

• The Land Before Time