In an act of biological terrorism, a trio of aliens try to poison the ventilation system of a suburban shopping mall. When their plan fails, one of the aliens is forced to abandon his body and take over the body of a pregnant woman named Nancy Salvo. However, the violence of the absorption proves too much for Nancy's body, which promptly goes into labor. While Nancy is rushed to the hospital, the two remaining invaders follow the ambulance to retrieve their comrade -- and the baby she is about to deliver!
When the aliens transmit the hospital events to the Advocacy, Norton picks up the radio impulses and the team sets out to investigate the latest alien plot. Meanwhile, Nancy gives birth to a baby boy. But when her unsuspecting husband tries to see her, Nancy brutally murders him, leaving a hideously disfigured body with the three-fingered alien trademark. After one look at the corpse, Harrison and Ironhorse take the newborn out of the nursery for safekeeping.
As Suzanne begins to study the baby, she makes a chilling discovery -- the child possesses a combination of human and alien tissues and is, quite literally, a half-human, half-alien mutant. Suzanne and Harrison are further stunned when, in the matter of a few hours, the child matures from a day-old infant to a strapping three-year-old toddler!
Knowing that the baby could hold the key to their survival in the earth's atmosphere, the aliens return to the hospital to kidnap the little boy, whose powerful urge to "recombinate" with his mother has led him to kill his nurse and escape. When Nancy is finally reunited with the half-breed son, she refuses to let her alien cohorts use the child for experiments. In the interest of their cause, the two aliens murder Nancy, while the little boy flees the scene. Before long, the team and the aliens track down the mutant child and meet in a hostile face-off. Just as the aliens take aim at Suzanne and Harrison with their weapons, Ironhorse and his special forces come up from behind and kill the aliens. After the showdown, the alien child undergoes a strange metamorphosis, becoming the day-old human infant once more.
A month later, Suzanne announces that the baby is completely normal and the team releases him to his grandparents.