In a quiet village in 1906 Vermont, Helen, Dorothy McGuire, a mute girl, attends a silent film screening in a local inn. During the screening, a lame woman limps out of the theatre to her room and is strangled by a man who was hiding in her closet. Her murder is the third in a string of killings that have unsettled the community, and the killer seems to be targeting women with disabilities.
Dr. Parry, Kent Smith, a friend of Helen's, drives her to the Warren home, a sprawling estate outside town where Helen is employed as a live-in companion for the bedridden Mrs. Warren, Ethel Barrymore. Also residing in the mansion are Mrs. Warren's stepson Albert, a local professor; her son Steven, a charming, rakish playboy; and the live-in staff: Mrs. Oates, Elsa Lanchester, a housekeeper; her husband Mr. Oates, Rhys Williams, a handyman; Blanche, Albert's secretary who is having an affair with Steven; and Nurse Barker, whom Mrs. Warren verbally abuses.
In the mansion, while Helen pauses to see herself in a mirror, the killer begins spying on her. Before anything happens to Helen, someone enters the area. Helen finds Mrs. Oates and they discuss the murder, with Mrs. Oates expressing fear for Helen as the killer appears to be targeting women with disabilities. After Mrs. Warren loses consciousness, Dr. Parry is summoned to the home. Nurse Barker discovers that the bottle of ether has gone missing, and Albert sends Mr. Oates to fetch some in town. Meanwhile, Mrs. Warren regains consciousness and urges Dr. Parry to take Helen with him. He offers to take Helen to Boston and help her work through the trauma of her parents' deaths, which triggered her muteness. She agrees, and Dr. Parry makes plans to return in the evening.
After an argument with Steven, Blanche asks Helen if she can leave with her that night. She agrees, and Blanche goes down the spiral staircase to the basement to retrieve her suitcase. Blanche is attacked and strangled by the killer. Helen finds her corpse in the basement and is confronted by Steven. Frightened that he is responsible, she locks him downstairs and flees upstairs. She attempts to wake Mrs. Oates, who has passed out, drunk on the professor’s stolen brandy. Helen tries to call Dr. Parry but cannot speak to the telephone operator.
Albert finds Helen frantic, and she writes on a notepad that Blanche has been murdered. As he follows her up the staircase to Mrs. Warren's room, he confesses to killing Blanche out of jealousy: “Blanche, whom I loved, did not love me.” He reveals how he got everyone out of the way to get her alone, and that he has a goal of killing the “weak and imperfect of the world.” He adds, “I saw you earlier looking at yourself in the mirror, and you had no mouth.” > Blanche, whom I loved, did not love me.
Helen flees, locking herself in Mrs. Warren's bedroom. The constable arrives and is answered at the front door by Albert; he leaves a message for Helen that Dr. Parry cannot return that night. Helen returns to the basement to free Steven but finds Albert waiting. He chases her, but they are confronted by Mrs. Warren, who is armed with a gun and shoots Albert, killing him, and Helen screams in horror. Mrs. Warren explains that she had suspected Albert, but the murders had stopped when Steven was away; they resumed when Steven returned to deflect suspicion onto him.
Mrs. Warren orders Helen to retrieve Steven, freeing him from the basement closet. She embraces him and dies on the staircase in his arms. Downstairs, Helen calls Dr. Parry on the telephone—she is now able to fully speak.
