In this psychological thriller, a very pregnant Francesca (Patty Duke) travels from Los Angeles to the small town of Dulwich, Minnesota to meet her late husband's mother, Mrs. Kinsolving, whom she has never met before. Upon arriving at the Kinsolving Manor, a large mansion that sits on the outside of town, Mrs. Kinsolving (Rosemary Murphy) is cold to Francesca. She questions whether Francesca is actually pregnant with her son's baby. She also tells Francesca that she wants nothing to do with her or her baby in the future. It soon becomes clear that Francesca cannot leave that night as a blizzard has made the roads impassable. Francesca is forced to stay in the Kinsolving mansion for a few days. She soon begins to suspect that something is amiss due to inconsistencies in information between what her late husband (Matthew) told her and Mrs. Kinsolving's statements to her.
While Matthew never mentioned he had a sister, Mrs. Kinsolving claims that the mentally challenged and non-verbal Kathleen (Sian Barbara Allen) is Matthew's sister. After Mrs. Kinsolving retires for the night, Francesca sneaks around and discovers in the family bible that Matthew's mother (whose name is Maria) died 11 days after Matthew was killed in the Vietnam War out of shock after receiving the death certificate. Mrs. Kinsolving is actually Matthew's aunt, Maria's sister, Katherine, who is Kathleen's mother and the mother of Kenny (Richard Thomas), a serial rapist and murderer who is hiding somewhere in the Kinsolving mansion a fugitive from the local authorities.
The following day, Francesca goes into labor, but Mrs. Kinsolving refuses to call for an ambulance. She sedates Francesca heavily. When the baby is born, Mrs. Kinsolving announces it is dead and hands the baby over to Kathleen to bury.
That night, Kathleen rouses Francesca and takes her to the attic where she finds Kathleen has hidden her baby in a picnic basket. The baby is very much alive. Mrs. Kinsolving, suspecting Francesca is sneaking around the mansion, locks her in her room. Kathleen is able to locate the key to the room and unlocks it, allowing Francesca to care for her baby.
Another night later, Francesca secretly spies the unsuspecting Kenny who is hiding in the basement laundry room. She overhears his conversation with Mrs. Kinsolving, and it is menacing. Meanwhile, Mrs. Kinsolving discovers that the family bible has been opened to the page detailing the date of Maria Kinsolving's death. Mrs. Kinsolving informs Kenny that Francesca knows Maria is dead, but does not think she is aware that Kenny is hiding in the mansion. The next morning, Mrs. Kinsolving announces that the blizzard has cleared enough for a driver to take Francesca into town to take the bus back to Los Angeles.
At breakfast, the driver arrives.... and it is Kenny. Francesca quickly tells Mrs. Kinsolving that she left her gloves in her third floor room and she needs to retrieve them. Instead she gets her baby from the attic, hides the baby under her coat and flees the mansion. However, Mrs. Kinsolving spots Francesca running away and yells for Kenny to get her. He takes chase and Kathleen notices.
Francesca sees Kenny is quickly gaining ground, and she darts into the carriage house in an attempt to elude him. He locates her, they struggle, and he knocks Francesca unconscious. The baby slips from under her coat. Kenny smiles sadistically and covers the crying baby's face with his hand. Suddenly, Kathleen sneaks up behind him and stabs him in the back with a pair of scissors. The film ends with Mrs. Kinsolving cradling her dead son as Kathleen and Francesca, holding her baby, look on.