In Los Angeles, Frank Davis and his wife Lenore are expecting their second child. Frank is a successful public relations consultant and his wife is a stay-at-home mom for their first child, Chris. The couple avoided having a child for several years while Lenore took contraceptive pills. When their child is ready to be born, they leave Chris with a family friend, Charley, and go to the hospital. Their second child, a baby boy, is born monstrously deformed, with fangs and claws. Immediately after birth it kills the doctors and nurses in the delivery room and flees through a skylight. Lenore is left alive, screaming for her child as a horrified Frank discovers the carnage.
Frank and Lenore are allowed to leave the hospital while the police, including Lt. Perkins, investigate the killings. Frank and Lenore receive attention from the press, which results in Frank being fired from his job at a public relations company. Meanwhile, the Davis’ baby lurks around outdoors, killing several people, including a milkman. As the killings continue, the press and the police hound Frank and Lenore. Frank meets with medical researchers who convince him to sign documents allowing them to experiment on the child’s body once it has been found and killed. Frank denies that the child is his son and joins the hunt for the murderous infant.
The doctor who prescribed the contraceptive pills to Lenore is contacted by an executive of a pharmaceutical company. The executive acknowledges that the Davis’ child’s mutation may have been caused by the drugs. He tells the doctor that the child must be destroyed to prevent discovery of the company’s liability. Meanwhile, the child makes its way to a school. Frank learns of the child’s location and arrives at the school, where police officers are present. Frank informs Lt. Dixon that Chris attends the school. The baby attacks and kills an officer in a classroom before escaping through a window into the night.
Later, Frank discovers that Lenore is hiding the infant in the basement of their home. Chris runs away from Charley’s house in order to get back home, and Charley drives after him. Lenore pleads with Frank and promises that the baby would not hurt their family. Frank, armed with a gun, enters the basement, where he finds Chris talking to the baby and promising to protect him. Frank shoots at the baby, injuring it. The infant flees the basement and attacks Charley, biting him on the neck and killing him.
The police track the infant into the sewers, where Frank hunts him with a rifle. When he finds the baby, he realizes that it is frightened. He apologizes to the child and picks him up. Wrapping the baby in his coat, Frank tries to elude the police, but a mob of armed cops confronts him as he exits the sewers. He pleads for them to study the child, but to not harm him. A fertility doctor shouts at the police to kill him. The child suddenly leaps from Frank’s arms and attacks the doctor as the cops open fire, killing both the infant and the doctor. As the Davises are escorted away by the police, Lt. Dixon receives news that another deformed baby has been born in Seattle.