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 orange T. Rex named Rex (John Goodman) playing golf. 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. 65 million years in the past in the Cretaceous Montana, Rex was a savage and frightening creature who terrorized smaller dinosaurs. As he is seen chasing 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 purple 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: Dr. Julia Bleeb (Julia Child), a scientist from the Museum of Natural History who will aid the dinosaurs, and Professor Screweyes, Neweyes' insane brother. He instructs them to find Bleeb and avoid Screweyes.
Neweyes drops the dinosaurs off at the East River in 1993, where they meet a young 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 (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 for 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 "Roll Back the Rock (To the Dawn of Time)" (the movie's theme song). When he sees the Apatosaurus balloon coming out in the parade however, Rex mistakes it for being real and hand shakes it too tight with his claws, causing its air sealer to pop open. The balloon runs out of air and falls on the dinosaurs, who are otherwise unharmed. 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 (Kenneth Mars) is running his "Eccentric Circus". They meet Professor Screweyes and cite their desire to join his circus. As they sign a contract, 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; reluctantly and sadly, the dinosaurs accept the offer.
Believing that his friendship with Louie and Cecilia would be lost, Rex, reverting them back to their human selves (and thus wearing off the effects of the Brain Drain), tells the two to remember him.
As the kids 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 the 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, and many run away. However, a crow accidentally activates flare lights, breaking Rex out of the trance and causing the hypnotic effect to end. The savage Rex realizes he has been tricked, becomes enraged and tries to attack Screweyes. However, Louie steps in and desperately tells Rex that killing Screweyes will not be worth it; 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, possibly sparking a relationship between the two. After Stubbs arrives to resign from Professor Screweyes's employ (which includes him giving him back most of his belongings with comedic results), Neweyes, Louie, Cecilia and the dinosaurs board the aircraft, leaving Screweyes to be swarmed upon by the crows. When the crows take off, all that is left of Screweyes is his fake eye.
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.
Why it Sucks
- Annoying characters.
- Poorly written script.
- Overuse of close-ups on the characters' eyes.
- Somewhat misleading title: While Rex and the other dinosaurs are the center characters of the film, they aren't the main characters as there more of the supportive characters, The real main characters are Louie and Cecilia, who both don't even appear until later on in this film.
- There are low quality scenes such as Stubbs giving Professor Screweyes almost all of his belongings that drags on way too long and an unconvincing scene of Rex singing.
- Inconsistent tone, as the first half of the movie is a generically happy animated movie about two kids meeting dinosaurs and coming to New York City, but when it comes to the main villain's plot in the second half, it suddenly changes into a dark sci-fi/horror animated movie, then near the end of the film after the villain is defeated, it goes back to to a more kid friendly tone like it was early.
- Kind of inappropriate moments in a kids movie such as:
- Professor Screweyes making his customers signing in blood.
- Cecilia acting strangely sassy while telling Louie that he should kiss her, due to her being around 12 years old.
- Scary scenes such the demise of professor Screweyes and a sequence featuring the dinosaurs temporarily returned to their savage state.
- Re-used animation of the green alien, Vorb, telling the captain to go.
- Great animation.
- The voice acting is spot-on.
- There are a couple of funny scenes that make this film somewhat likeable.
- The idea of a story about dinosaurs going forth in time to make children happy is interesting.
- Captain Neweyes' last line "And that's the way it is" is a reference to his voice actor, Walter Cronkite, who used it as a sign-off phrase on television. Years later, Walter Cronkite would voice Benjamin Franklin on the animated TV series Liberty's Kids.