In the town of Elkerton, Marvin Mange is an evidence clerk at the local precinct who dreams of being a police officer like his late Father. However, his repeated failure of the officer's physical test prevents him from becoming one. Due to Marvin's meek nature, he receives little respect from the populace and is especially tormented by his superior, police sergeant Doug Sisk. Marvin is also smitten with environmentalist Rianna Holmes, but fumbles a first encounter with her.
While alone at the station, Marvin receives an emergency call. With no officers available for dispatch, Marvin responds to the emergency himself, getting into a freak car accident that sends him tumbling down a mountain. The grievously injured Marvin is discovered by mad scientist Dr. Wilder, who saves his life by means of assorted animal organ transplants. Marvin finds himself back at his neighborhood days later and resumes normal life, discovering that he can now perform extraordinary physical feats and possesses keen animal-like instincts. While visiting his friend Miles at an airport, Marvin sniffs out and apprehends a man attempting to smuggle drugs through the terminal. Marvin's assistance in the arrest garners media attention and he is promoted to full-fledged police officer assigned under Sisk.
Over the course of several days, Marvin awakens outside of his home from bouts of sleepwalking, hearing subsequent reports of attacks in the night being attributed to a savage 'animal'. Wilder introduces himself to Marvin, cautioning him of his rising animalistic urges as side-effects of the operation. Despite continued predicaments and embarrassments caused by his instinctual behaviour, Marvin's abilities allow him to excel as a police officer and bond with Rianna. However, Marvin is questioned one day after another attack, where a police sketch implicates Marvin as the animal. Marvin is subsequently placed on paid leave.
Afraid of what he has seemingly become, Marvin barricades himself inside his home. Rianna arrives to comfort Marvin and the two spend the night together. The following morning, the police raid Marvin's home, whom they suspect of attacking a hunter the night prior. Marvin escapes and flees into the woods. An armed mob headed by Sisk is formed to hunt Marvin down, while Marvin's friends Miles and Fatty seek to warn him. Marvin encounters Wilder who confides the existence of another patient - who had undergone the same procedure - whom he suspects is the true culprit for the hunter attack.
Sisk's lone pursuit of Marvin results in a near-fatal fall into a chasm, but he is saved by the latter. In spite of the rescue, Sisk holds Marvin at gunpoint, where he is suddenly killed by Rianna; revealing her as Wilder's other patient. Rianna confesses that she was responsible for the hunter attack, justifying her actions in protecting an orphaned turkey vulture which she reared and released back into the wild. The mob arrives, threatening to kill Marvin. Miles makes a false confession to being the animal, to protect Marvin and Rianna. Due to the heavy implications in prosecuting Miles, a Black American, the mob are suddenly reluctant to act and casually pardon him.
One year later, Marvin and Rianna have married, starting a family and open an animal sanctuary. While watching television, they witness Dr. Wilder being awarded the Nobel Prize for his transplantation procedure.
