A scientist doing experiments on a human fetus discovers a method to accelerate the fetus into a mature adult in just a few days. However, the "adult" fetus turns into a homicidal psycho and looks for a new formula to prevent her from aging further.
A pregnant mother is involved in a horrific car accident. She survives long enough to be taken to hospital where the still undeveloped fetus is retrieved and contained within an artificial womb. The womb instigates a rapid growth rate which causes the child to grow into adulthood within a matter of weeks. All is not well though as the child begins to exhibit some horrific tendencies...
- Dr. Paul Holliston (Rock Hudson) accidentally strikes a Doberman pinscher with his car, and treats the injured dog in his home laboratory, assisted by Martha Douglas (Diane Ladd), his sister-in-law. Paul discovers that the dog is pregnant, and hopes to save the puppies, if not the mother. He calls his son, Gordon (John Elerick), asking him to bring supplies from the research hospital.
Gordon is about to make love to his wife, Helen (Anne Schedeen) , who is pregnant with their first child. This is the first they have heard from Paul since his wife died in an automobile accident.
Gordon and Helen deliver the supplies, and are happy to see Paul back at work. Paul is equally happy to see them, and is looking forward to having a grandson. He goes upstairs to change his clothes, and talks wistfully to a photograph of his deceased wife, Nicole. Martha, standing in the doorway, says that Nicole is still with them. Paul reminds her that the accident was his fault, and returns to the lab to find that one of the pups has already died.
After Gordon and Helen leave, he considers a new strategy. Within a few hours, the mother is dying and only one fetus remains, but he is able to accelerate its development with a growth hormone called placental lactogen, a finding he records on tape. Soon the puppy, which Paul names Number One, is able to live outside the womb. He keeps her tranquilized in an isolette while he continues to experiment with the growth hormone. In less than three days, Number One is a fully-developed puppy; she is soon full grown and the tranquilizers are discontinued.
Paul records the dog's remarkable ability to learn, and determines that she is experiencing no negative side effects. Believing he can rescue miscarried babies, Paul wants to experiment on a human fetus and arranges a meeting with Dr. Jim Wiston (Jack Colvin). When Paul leaves Number One alone in the car, she lets herself out against his orders. She is accosted by a small terrier, which she kills and hides before returning to the car.
After Paul convinces Jim to supply him with a twelve-to-fourteen-week-old human fetus, Jim procures a female fetus from a suicide victim, which Paul hopes to grow to five weeks of age. Soon the fetus is aging one month per day; Paul discontinues the hormones, tranquilizes the baby and places her in an isolette. However, she continues to grow and is aging at the rate of two years per day, with old cells dying rapidly as new cells develop. Paul tries several methods to halt the process, and finally succeeds with the addictive drug, methotrexate. As he weans the girl from tranquilizers, Paul educates her subconsciously by playing recorded arithmetic lessons. When she awakens, Paul names the girl Victoria (Barbara Carrera) and tries to communicate with her by reciting equations. Paul is overjoyed when she is able to recite the equations on her own.
The next day, Paul convinces Martha to stay with Gordon and Helen, using the excuse that he will be in Denver, Colorado. As Victoria explores the house, she removes a fish from Paul's aquarium and watches it die. Number One, having formed an instant bond with Victoria, hides the fish.
Paul records Victoria's progress, noting that she utilizes virtually all of her brain's intellectual capacity, unencumbered by the experiences of childhood. He is also satisfied that she has not become addicted to methotrexate. Paul's only remaining concern is Victoria's slow emotional development, so he has her read the Bible to give her a moral foundation, but she finds the book illogical.
Paul tells Victoria how she came to be, and that she was given her name because her survival was a victory. He also tells her that, had he not been able to control her cell growth, she may have lived and died without any awareness, or could have been nothing more than a medical curiosity.
One day, Martha, believing that Paul is still in Denver, returns to the house on an errand. With Number One following her menacingly, Martha searches for Paul in the lab, while Victoria hides behind the door with a knife until Martha leaves. Victoria feels isolated and asks Paul to show her "the outside world," so he takes her on a two-week vacation up the California coast.
Upon their return, Paul and Victoria visit Gordon and Helen, who are having a party. Paul introduces Victoria as his new assistant, a University of Colorado graduate. Some guests are charmed by Victoria, including a flirtatious computer programmer named Collier (Vincent Baggetta); some aren't, particularly chess champion Frank Riley (Roddy McDowell), whom she humiliates in her first chess game. Helen adores Victoria, but Martha takes an immediate disliking to her.
Later that night, after Paul and Victoria consummate their relationship and have sex for the first time, she is at first joyful, then suddenly racked with pain. Upon realizing that her cells are again aging and dying rapidly, Victoria injects herself with methotrexate. The next day, she consults Collier's medical computer for a permanent cure, and learns that she needs the pituitary gland extract from a five to six-month-old fetus.
Victoria sits on the beach and ponders her dilemma, but is soon playing with a group of children, until the pain returns and she needs another dose of methotrexate.
Later, at a dinner party, Victoria learns that Helen is more than five months pregnant. Martha tells Paul that Victoria is a fraud and one of them has to leave. Victoria overhears and, while Martha is asleep, gives her an injection. Martha leaves for Minneapolis, Minnesota, the next morning, and dies of a heart attack within hours.
While Paul travels to Minneapolis, Victoria learns how to perform a cesarean section, destroys Paul's tape-recorded notes, and breaks into the office of Helen's obstetrician, where she selects a victim from his patient files: a prostitute who is six months pregnant. Victoria performs the procedure of removing the prostitute's fetus in Paul's laboratory, but despairs when she discovers that the fetus is dead, until Helen pays a surprise visit.
In Minneapolis, Paul learns that Martha's death was caused by a toxic chemical, and, deducing that Victoria is the culprit, contacts Gordon, who is in Seattle, Washington, on business. When they arrive at Paul's home, they call an ambulance upon finding the prostitute, then run to the laboratory, where Victoria has already removed the fetus from Helen's body. Victoria begs for Paul's understanding, but he now sees her as a monster and injects her with a large dose of placental lactogen. She in turn stabs Gordon, who dies trying to save his wife and child.
As the ambulance arrives, Victoria drives away, with Paul in pursuit. After a car chase, he runs her off the road, causing her car to crash and burn on the beach. Victoria, who has aged dramatically, escapes the wreck. When Paul tries to drown her, police and paramedics come to her rescue and arrest Paul. The medics discover that she is about to give birth. The baby is delivered and taken away as Victoria dies. Paul, now in custody and taken away in handcuffs to a nearby police car, screams in horror... unable to stop yet another evil monster he has created.