New York heiress Celia Barrett, Joan Bennett, and her lawyer Bob Dwight, James Seay, are on the verge of marriage, but he wants her to be sure, so he persuades her to vacation in Mexico with her friend Edith Potter, Natalie Schafer.
While there, Celia falls in love with and marries American architect Mark Lamphere, Michael Redgrave.
On their honeymoon, after she playfully locks him out of their hotel room, he turns cold toward her and abruptly leaves, claiming that he has urgent business at home and will make arrangements for her to follow.
Five days later, Celia arrives at Mark's hometown of Levender Falls. Mark is in the city on business, so it is his sister Caroline Lamphere, Anne Revere, who meets Celia at the train station and takes her to the family estate, Blade's Creek.
Celia is surprised to learn that Mark is a widower with a teenaged son, David Lamphere. The boy is being looked after by Caroline and by Mark's secretary, Miss Robey, Barbara O'Neil, who wears a scarf to conceal the scar on her face from a past rescue. Miss Robey’s hidden history adds a tense undercurrent to the household.
Caroline hints that Mark is somewhat responsible for his first wife Eleanor's death; David, estranged from Mark, says that his father murdered her.
At a party, Mark shows the guests six rooms he built in a wing of Blade's Creek, each a reconstruction of a notorious murder scene, all with female victims. A seventh room is locked, and Mark refuses to show it.
Miss Robey accidentally reveals her full face to Celia and admits that she had plastic surgery to remove the scar. The family keeps up the pretence to spare Robey’s job, and Celia realizes that Miss Robey resents her for marrying Mark.
Mark's behavior toward Celia grows increasingly erratic. She secretly enters the seventh room, which is a replica of her own bedroom, and begins to fear that Mark intends to murder her.
Although Mark loves Celia, he harbors a recurring urge to kill her, which he struggles to suppress. Celia returns because she loves him and wants to help, but the house feels claustrophobic — Caroline is taking David to New York, Miss Robey has been fired, and the servants are away.
To protect Celia, Mark leaves, yet his compulsion draws him back. He finds Celia waiting for him in the seventh room — she would rather die than live without him.
He explains that the childhood trauma behind his homicidal tendencies is triggered by lilacs and locked doors. As he speaks of being locked in his bedroom as a boy, they hear a door slam shut, sending him into a homicidal trance. Celia explains that it was not his mother but his sister who locked the bedroom door, and he regains his senses. They notice smoke: Miss Robey, thinking Celia is alone, has locked them in and set fire to the house.
Mark bursts through the door and they escape. They resume their honeymoon in Mexico, and Mark tells Celia that she has helped quell the root of the evil in him.
