Primatologist Davis Okoye was once a military spec-ops agent who rescued an albino baby gorilla named George by exterminating poachers after his family was butchered by them during a routine patrol in an anti-poaching operation in Africa. Believing that George cannot survive on his own, Davis took the baby ape under his wing.
In his military years Davis had seen too many evils in men, in contrast to what he perceives as animals' honesty, so years later he quit the military. Davis then went onto become a primatologist and learned everything he needed to know about primates. During this time as a primatologist Davis taught George sign language.
One day, three biohazard containers belonging to EnerGyne that house experimental substances. Substances within the three containers possess capability to mutate animals upon inhalation. One of the three containers crashed through the cage where Geroge was kept. George inhaled the substance and began to mutate. Elsewhere, two other containers dropped into swamp and forest that turned a wolf and a crocodile into terrifying giants.
Okoye receives an emergency call from his peers, upon receiving report that George broke out of his confinement and killed a grizzly bear in a turf dispute. Okoye eventually reach George, Okoye is astounded to see that George had grown at least three times his own size overnight. Desperate to look for answers, Okoye teams up with scientist Kate Caldwell, who was kept away from seeing her sick brother by the diabolical businesswoman Claire Wyden. When George eventually broke out of his new confinement and things inevitably go awry, Okoye and Kate work together to try to stop George and the monsters from wreaking havoc on Chicago.
Eventually, Okoye stands up to Wyden before being shot. Luckily, he and Kate trick the villainess into getting eaten by George after they put a healing serum in her bag. The mutated wolf and crocodile Ralph and Lizzie attack the city, but George, aided by Okoye, defeats them and seemingly dies. As it turns out, George was pretending to be dead to make Okoye feel sorry for him, but he and Davis become heroes.