In a village in 1906 Vermont, Helen (Dorothy McGuire), a mute girl, attends a silent film screening in a local inn. During the screening, a paraplegic woman limps out of the theatre to her room. She is strangled by a man who was hiding in her closet. Her murder is the third in a string of killings in the community.

Dr. Parry (Kent Smith) drives Helen to the Warren home, a large estate outside town where Helen is employed as a livein companion for the bedridden Mrs. Warren (Ethel Barrymore). Also residing in the house are Mrs. Warren's stepson Albert (George Brent), a local professor; her son, Steven (Gordon Oliver), a charming playboy; and livein staff: Mrs. Oates (Elsa Lanchester), a housekeeper; her husband Mr. Oates (Rhys Williams), a handyman; Blanche (Rhonda Fleming), Albert's beautiful secretary who is having an affair with Steven, who has recently returned from abroad; and Nurse Barker (Sara Allgood), who Mrs. Warren verbally abuses.

Mrs. Oates discusses the murder and expresses 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 retrieve 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' death, the shock of which triggered her muteness.

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.

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 her out of jealousy. Albert then 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." Helen flees, locking herself in Mrs. Warren's bedroom.

Meanwhile, the Constable (James Bell) shows up. Helen returns to the basement to free Steven but finds Albert waiting. He chases her, but Mrs. Warren shoots Albert, killing him, and Helen screams in horror. Mrs. Warren explains that she suspected Albert was the killer, but wasn't sure because the murders stopped when Steven was away. She notes Albert started killing again when Steven returned to cast suspicion on him.

Mrs. Warren orders Helen to retrieve Steven, so she frees him from the basement closet. Mrs. Warren 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.

A 1946 American Black & White psychological horror film directed by Robert Siodmak, produced by Dore Schary, screenplay by Mel Dinelli, based on Ethel Lina White's novel "Some Must Watch" (1933), cinematography by Nicholas Musuraca, starring Dorothy McGuire, George Brent, Ethel Barrymore, Kent Smith, Gordon Oliver, Rhonda Fleming, Elsa Lanchester, Sara Allgood, Rhys Williams, James Bell, and Carlton the Bulldog as Himself.

The closeups of the killer's eyes and hands are those of director Robert Siodmak.

Ethel Barrymore received an Academy Award nomination for Supporting Actress.

The silent film in the cinema at the beginning is D.W Griffith's "The Sands of Dee" (1912), starring Mae Marsh and Robert Harron.

There are several major differences from the novel. The maid stalked by the killer was not mute. It was set in contemporary England, not early 1900's New England. Finally, the title of the film and the idea of incorporating a spiral staircase as a thematic element comes from another source entirely: Mary Roberts Rinehart's 1908 novel "The Circular Staircase." The heroine of the book was not mute or crippled, nor were any of the murderer's victims.

A good many plot elements in this film were picked up and used in the February 15, 1965 episode of The Alfred Hitchcock Hour, titled "An Unlocked Window."

This was one of RKO's biggest hits of the 1940s.

RKO Pictures acquired the rights from independent producer David O. Selznick, along with several others he owned, in order to help Selznick finance the Western "Duel in the Sun" (1946). Under the terms of the sale, Selznick was given a back end cut of the film's earnings, and subsequently gave star Dorothy McGuire a convertible as a bonus for appearing in the film. The screenplay originally bore the same title as White's novel, though a subsequent working title for the project was "The Silence of Helen McCord."

The plot keeps the tension until the very end in this eerie and atmospheric, heartpounding, suspenseful stylish film with magnificent Black & White cinematography that follows the German Expressionism, and wonderful set decoration in Victorian style. If you like film noir, thrillers, or subtle horror, this cinematic perfection is a mustsee.