A serial killer is terrorizing a community, stalking and killing five young women, each with a physical disability. The ongoing investigation is led by Lieutenant Sam Wanamaker Fields, a steady presence in the tense pursuit of the culprit. At the center of the mystery stands Jacqueline Bisset as Helen Mallory, a striking young woman who has been unable to speak since witnessing the death of her husband and daughter in a devastating house fire. Her silence has left her vulnerable, and her doctor urges caution; John Ronane plays Dr. Rawley, the medical examiner who advises Helen to stay with her elderly, invalid grandmother, Mildred Dunnock as Mrs. Sherman, in order to avoid becoming a potential sixth victim.
Inside the family mansion, Helen encounters her enigmatic uncle, Christopher Plummer as Joe Sherman, a respected psychiatrist who exerts a quiet authority over the sprawling estate. The household is layered with intrigue: Blanche, a glamorous Southern belle played by Gayle Hunnicutt, serves as Joe’s secretary and lover and is simultaneously entangled with Joe’s younger brother, Steven, portrayed by John Phillip Law. The atmosphere thickens as a late-night storm unfolds and a mysterious man is spotted on the mansion grounds. When Lieutenant Fields and another officer confront him, a shot is fired and the assumed intruder is killed, but the twist is quick to reveal that this figure is not the true killer.
Unknown to Helen, the real killer continues to shadow her every move, weaving through the mansion’s corridors with unsettling patience. In the tense sequence that follows, Joe gives Oates, the handyman, the task of repairing the generator in case the power fails, a setting that will prove crucial when the lights go out and the house becomes a trap. Mrs. Sherman nearly slides into a diabetic crisis and Nurse Baker, Elaine Stritch, administers insulin to steady her condition. Dr. Rawley returns to check on Mrs. Sherman and, sensing Helen’s fragility, reassures her guardians of Helen’s safety while he departs. During a moment of dark intimacy, Oates consoles Helen, but tragedy strikes when he is killed outside the gate, strangled in the shadow of the mansion’s perimeter.
Over dinner, Steven drops a bombshell: he and Blanche plan a Caribbean getaway to Tobago, a revelation that Blanche receives with anger because she refuses to join him on the trip. Joe withdraws in disappointment, while upstairs he inspects the cellar’s generator, a routine task that becomes ominously symbolic. Blanche reveals a troubling truth, and soon after she is found strangled, plunging the house deeper into fear. Helen learns that both Oates and Blanche have been murdered, and the web of suspicion tightens around the people she thought she could trust.
The night’s events propel Helen into action: after Steven disposes of Oates’s body, he descends the spiral staircase toward the cellar, where Helen, fearing the killer may strike again, locks him inside. She scrambles to write a note for Joe, warning him of the peril closing in on them. Yet the killer’s identity is not what it seems. Joe is revealed as the true murderer, and as Helen’s voice remains unheard, Steven finds himself trapped by his own family’s secrets. Lieutenant Fields appears briefly, arriving with the insulin needed for Mrs. Sherman, while Dr. Rawley calls to say he will come in the morning to retrieve Helen. The storm rages on as Helen flees into the rain to gain time and safety.
Desperate to escape, Helen returns to the cellar to attempt a rescue of Steven, but Joe corners her with deadly intent. The struggle culminates when Joe is shot twice from behind by his own mother, Mrs. Sherman, ending his murderous arc in a decisive act of intergenerational justice. Steven manages to free himself and rushes to assist his mother. With her voice finally returning, Helen makes a crucial call to Dr. Rawley and, in a quiet yet powerful moment, declares the truth of her restored speech: > It's...it's Helen.
