Set in rural New Jersey in the early 20th century, Margaret Randolph, a young doctor of psychiatry, arrives at the palatial estate of Dr. Roland Maradick and his wife Charlotte. She has been sent by her mentor Dr. Richard Brandon, a colleague of Dr. Maradick’s, to observe the psychological condition of Charlotte and make notes to help determine if Charlotte will need to be committed to Bellevue Sanitarium in New York. The Maradicks' maid, Rebecca Peterson, answers the door and, speaking in a Southern dialect, expresses surprise and suspicion at Margaret’s appearance, as Dr. Brandon was actually expected. Reluctantly, Rebecca lets Margaret into the home. Once inside the grand Mural Gallleria of the home, Rebecca tells Margaret that the estate belonged to Charlotte’s late first husband and that it was left to their daughter Dorothea. After she goes to fetch Dr. Maradick, a small red ball rolls down the grand staircase to Margaret’s feet. Margaret picks it up and, assuming it belongs to Dorothea, tries in vain to coax the girl out. Rebecca returns and, during the ensuing conversation, discloses that she met Dr. Maradick at the race track and that he offered her the job as the maid when the previous help retired. He also offered to move her ailing sister, who still lived where they grew up in the South, up to the house for treatment. When Margaret gives Rebecca the ball, Rebecca acts surprised and wonders where it came from.

Dr. Maradick enters and asks Rebecca to check on Charlotte. She leaves, taking the ball with her. He expresses concern that Margaret is a novice. Margaret reassures him that Dr. Brandon sent her as his charge and, as such, may be able to expedite getting her admitted into Bellevue. Dr. Maradick is reassured and shares the details of Charlotte’s condition which include delusion and violent mood swings. He is forced to keep her sedated. A telephone ring interrupts their conversation, and Dr. Maradick ignores the call, saying that Rebecca will answer it. At the conclusion of their conversation, Rebecca appears saying that Charlotte is waking. As Dr. Maradick and Margaret climb the stairs to go to Charlotte’s bedroom, he trips on a child’s jump rope lying across the stair. In an angry voice, he gets up and claims that “she knows better.” He then apologizes for his outburst, and together he and Margaret head to Charlotte’s bedroom, taking the jump rope with him.

Upon arriving at the bedroom, Dr. Maradick enters first to confirm Charlotte’s condition. While Margaret waits outside, the red ball rolls out of the darkness from an adjoining to corridor. Margaret again tries in vain to coax Dorothea out. Giving up, she rolls the ball back into the darkness. Rebecca comes to fetch Margaret with the jump rope around her neck, and they enter the bedroom. Inside the bedroom Margaret observes Charlotte apparently in the throes of delusion, and Dr. Maradick and Rebecca are forced to strap her down. A phone call interrupts them again, and Dr. Maradick exits to take it. After Charlotte hurls insults to Rebecca, Margaret coaxes Rebecca to leave. Rebecca agrees with relief and takes the jump rope with her to put away. Once it’s just the two of them, Charlotte starts to appear more coherent, but also distrustful of Margaret. She insinuates that Rebecca is having an affair with her husband and believes Margaret is there to do her harm. When Margaret brings up Dorothea as a reason for Charlotte to accept mental treatment, Charlotte’s demeanor changes and she claims to want to recover and play with her daughter again. Dr. Maradick enters and is pleased that Charlotte appears to be acting rational and claiming to want to talk further with Margaret. Another phone call takes him from the room, and Charlotte asks Margaret for some time alone to rest and clear her head. She promises to eat and talk upon awaking. Sensing progress, Margaret agrees and leaves Charlotte alone to rest.

After exiting Charlotte’s bedroom, Margaret encounters Rebecca crying alone in a nearby Cloister Garden. She attempts to comfort Rebecca, believing that the young maid is distraught because of her mistress’s condition. But Rebecca surprises her by sharing that Charlotte is angry with her because Dr. Maradick coerced her into an affair and Charlotte caught them together one night. She also discloses that Dorothea had been ill, and Margaret surmises that that must be the reason for the girl’s shyness. Rebecca is surprised to learn that Margaret saw the red ball a second time. Charlotte abruptly appears in a wheelchair and orders Rebecca out of the room. Once Rebecca is gone, Charlotte discloses that Dorothea in fact died the previous month and that she is convinced Dr. Maradick poisoned her after she found hidden arsenic bottles in a waste bin. She believes Dorothea’s spirit is haunting the house because the red ball has rolled unexplained to Charlotte on many occasions, spooking Rebecca. But since the ball has been rolling to Margaret, Charlotte says she’s decided to trust Margaret to help her. Margaret believes all of this is another delusion, but Charlotte insists, telling Margaret that not only did her first husband leave his estate to Dorothea, but that in the event of Dorothea’s death, it all got left to Charlotte. Furthermore, in the event of Charlotte’s subsequent death, everything got distributed to charities. But no provision was made if Charlotte was committed. She believes Dr. Maradick wants only the estate because of gambling debts he racked up at the racetrack. She also claims to have proof with her attorney in New York. Taking a chance, Margaret offers to transport Charlotte to her attorney in order to verify Charlotte’s claims on the estate.

While trying to leave the estate, Margaret and Charlotte encounter Rebecca and Dr. Maradick, who block their exit. Dr. Maradick claims it all to be a delusion and asks Margaret to help him sedate Charlotte once again. Taking another chance, Margaret refuses to help and claims to know of the financial ramifications of institutionalizing Charlotte. Rattled, Dr. Maradick threatens to report Margaret’s actions to Dr. Brandon when the red balls rolls down the corridor. Rebecca screams and the phone starts to ring. Dr. Maradick runs from the corridor to get the phone, and Margaret, once again, goes to help Charlotte. A scream is heard from the adjoining Mural Galleria, followed by the sound of someone falling.

The three women run into the Mural Galleria to see Dr. Maradick at the bottom of the stairs with his head at an odd angle and blood pooling from it. Margaret examines him and discovers that he is dead. The women then see the child’s jump rope tangled up around his feet. Suddenly the red ball rolls out from the darkness near the front door. The front doors then open of their own accord. Charlotte thanks her daughter for her help, and then, from out of the ether, the sound of Dorothea’s voice is heard, acknowledging her mother and saying that she loves her. The doors then close by themselves, and the lights cut to black.