Graduate student Teri MacDonald (Helen Hunt) has trained a chimpanzee named Virgil to use American Sign Language. When her research grant is not renewed, she is forced to sell Virgil. Despite her being assured that Virgil will be sent to an animal sanctuary, instead he is taken to an Air Force base to be used in a top-secret research project involving platforms designed to simulate the operation of aircraft.
Airman Jimmy Garrett (Matthew Broderick) is assigned to the same chimp project; he begins to bond with Virgil and they become attached. Jimmy discovers that Virgil has been taught sign language. Jimmy informs his superior, Dr. Carroll (William Sadler), but Carroll is not interested.
Unbeknownst to Jimmy, the chimps trained on the flight simulators will be killed by radiation poisoning. Once they reach a certain level in operating the flight simulator, the chimps will be exposed to a lethal pulse of radiation in the simulator chamber to determine how long a pilot may survive after a nuclear exchange known as the second-strike scenario.
When a dead chimp is removed from the simulator, Virgil breaks away from Jimmy and escapes to the room adjacent to the vivarium. Virgil sees the dead body, returns to the vivarium, and screeches to the other caged chimps. Jimmy does not know that Virgil is communicating to the others of their impending peril.
Jimmy later becomes aware of the chimps' fate. He searches Virgil's file and phones Teri, who comes to the base. Concerned about his military career, Jimmy initially tells Teri that she should not have come and refuses to tell her anymore about Virgil's fate. Later, Jimmy goes to Teri's hotel and finds her as she is leaving. Teri tells Jimmy she is going to go to Washington to inform the National Science Foundation of the deception. Jimmy tells Teri that she does not have enough time because Virgil is scheduled to die soon.
Teri and Jimmy return to the base as Virgil is about to be placed within the flight simulator chamber for his final run. Jimmy challenges Dr. Carroll, the assembled military guests, and assorted politicians about the value of the project. Jimmy points out that the hypothetical pilot, knowing of the implications of the second-strike scenario, would know he is dying, and would, therefore, be affected by that knowledge. However, the chimps would not have the same awareness; thus, the project is flawed. This enrages Dr. Carroll, who promises Jimmy that his military career is finished.
Meanwhile, in the vivarium, some of the chimps have used a mop to get the keys to open their cages and have stacked crates and boxes in an attempt to escape through a skylight. Jimmy and Teri walk in to see the chimps escaping. Virgil, at the top of the stack, is about to break the skylight with a crowbar when the authorities enter. Goliath the chimp becomes very angry and fights with Dr. Carroll, who has obtained a cattle prod and is attempting to use it to foil the chimps' escape. The authorities are chased from the room. Goliath and Virgil end up in the flight simulator room, and a fire extinguisher is jammed, forcing the radiation generator in an exposed condition, potentially leading to an uncontrolled radiation blast. Jimmy gets Virgil and some other chimps out, but Goliath continues smashing the simulator and is caught inside. Jimmy and Virgil convince Goliath to yank out the extinguisher, but Goliath dies from radiation because the chamber cannot be opened until the radiation falls below lethal levels.
Jimmy and Teri steal a military plane to help the chimps escape, but they are stopped by military police. While the police are holding them, Virgil pilots the plane, and the chimps fly away. They eventually crash land in the nearby Everglades and evade a search. Just as the search is being abandoned, Jimmy and Teri see Virgil hiding in the bush with his chimpanzee girlfriend. Teri signs to Virgil that he and the others are now "free". The chimps disappear into the Everglades.Matthew Broderick as Jimmy Garrett
Helen Hunt as Teri MacDonald
William Sadler as Dr. Carroll (credited as Bill Sadler)
Johnny Ray McGhee as Robertson
Jonathan Stark as Sgt. Krieger
The film's producers were accused of animal cruelty by TV personality Bob Barker and the United Activists for Animal Rights. The American Humane Association, which consulted during production, filed a $10 million defamation lawsuit, arguing that the animal cruelty claims were based on hearsay. In 1994, over Barker's objections, his insurance company settled the lawsuit for $300,000.
Virgil was portrayed by "Willie", who now resides at Primarily Primates wildlife refuge. Willie was sent there along with Harry, who played his "girlfriend". They were retired from the entertainment industry and live with other males in a tightly knit group at the refuge in Bexar County, Texas.
The movie received generally positive reviews from critics. As of January 2013, it holds a 78% score amongst critics on Rotten Tomatoes.
In his review for The New York Times, Walter Goodman described the film as a "young folks' story, a sweet-natured boy-and-his-chimp tale (even the bad guys aren't all that bad - that's very arguable), with a dose of Animal Liberation to give the impression that something of current significance is going on."