Lenore Davis, portrayed by Sharon Farrell, awakens her husband Frank, played by John P. Ryan, in the dead of night to share the life-changing news that she is on the verge of giving birth. During their frantic journey to the hospital, they stop at the home of Charley, a family friend played by William Wellman Jr., to drop off their son Chris. As Lenore copes with the intense waves of labor pains, she expresses to Frank that this time around, it feels far more challenging than her previous experience. Once they reach the delivery room, Lenore is alarmed, insisting that something is terribly amiss, though the doctor reassures her that everything is progressing normally.
While Frank waits nervously in the hallway, he witnesses a horrifying scene as a nurse emerges from the delivery room, covered in blood, only to collapse nearby. Rushing inside, Frank discovers that a calamity has unfolded: everyone in the room has been killed except for Lenore. Hysteria overtakes her as she demands to know about their newborn, who has mysteriously vanished. As they are later questioned by detectives, they learn that their prior inquiries about abortion during Lenore's pregnancy have raised eyebrows. Frank attempts to clarify that they merely contemplated it briefly and ultimately wished to expand their family.
The following morning, a woman hears a mysterious infant's cries outside her home. Approaching the source of the noise, she is met with a horrifying revelation: the newborn's mouth reveals large fangs and claws, sending her into a state of panic. Due to the scandal generated from the disastrous events surrounding their child, Frank is soon terminated from his position at a public relations firm. After bringing Lenore back home from the hospital, he explains to Chris that he must remain with Charley for a while longer, fabricating a tale that the infant is unwell. Unfortunately, the mayhem continues as yet another murder is reported, this time implicating the baby in a gruesome act against a milkman, who meets his end while the child indulges in the milk from his truck.
In the midst of unraveling chaos, a pharmaceutical executive privately informs Dr. Norton, a local physician, of the need to suppress all knowledge of these incidents and destroy the baby's body. The rationale being that Lenore had once ingested a birth control pill produced by their company, which might have contributed to the child's horrific deformities. Dr. Norton later visits the Davis residence alongside a university professor, where they pressure Frank into relinquishing his rights to the baby’s remains, should it be captured. Upon their departure, Frank receives a call intended for Dr. Norton, learning that the baby has been located at a nearby school. There, he meets Lt. Perkins, who assures Frank that the authorities do not hold him responsible for his child’s actions. However, the mutant baby manages to elude capture, attacking an officer and disappearing into the surrounding woods.
As the sun rises, Frank discovers several milk bottles left at his doorstep and is puzzled by the sight of Lenore stocking the refrigerator with a plethora of food supplies. Although she yearns to have Chris back home, Frank insists that their son is better off with Charley. After Lenore's strange behavior raises Frank's suspicions, he uncovers that more than one milk bottle has been drained since he last checked—a troubling sign. That day, Chris escapes from Charley’s house and races home, while Frank, wandering through his house in a daze, confronts an empty crib. In an alarming turn of events, he is surprised by the sudden slam of a door behind him.
Chris, now in the basement, stumbles upon the family's deceased cat, leading Frank to grab a gun in preparation for confronting the unimaginable horror that has engulfed his family. When Charley appears seeking to help, the baby attacks, prompting Frank to instinctively shoot at his own mutant offspring. The baby manages to escape through a window, and as the police arrive, Frank pleads with Lt. Perkins to let him take part in the hunt to kill his monstrous child.
As night falls, the police determine that the baby has been navigating through the sewer system. Armed with a rifle, Frank, accompanied by several officers, ventures down into the dark underbelly of their town. Straying away from the others, he eventually finds the crying baby, who is pleading with its clawed hands. In a moment of empathy, Frank covers the child with his trench coat, lifting it into his arms. However, when they emerge from the sewer, they are met by a police barricade. Frank desperately implores the officers to spare the child's life, arguing that it should be studied and understood rather than killed. Tragically, when Dr. Norton orders them to fire, Frank instinctively throws the baby in a last-ditch effort to save it. In a grim outcome, the baby kills Norton, becoming the final victim in this string of horrifying events before the police shoot the infant down.
As they drive away from the scene, Lt. Perkins receives alarming news—another killer mutant baby has been born in Seattle, Washington, suggesting that the terror is far from over.
