The Uninvited

In 1937, London music critic and composer Roderick "Rick" Fitzgerald (Ray Milland) and his sister Pamela (Ruth Hussey) fall in love with Windward House, an abandoned seaside house, during a holiday on Cornwall's rocky coast. They purchase it for an unusually low price from Commander Beech (Donald Crisp).
Rick and Pamela meet Beechâs 20-year-old granddaughter, Stella Meredith (Gail Russell), who lives with her grandfather in the nearby town of Biddlecombe. Stella is deeply upset by the sale because of her attachment to the house, despite its being where her mother died. The commander has forbidden Stella to enter the house or to see Rick. However, she gains access to Windward House through Rick, who has become infatuated with her.
The Fitzgeralds' initial enchantment with the house diminishes when they unlock an artist's studio where they feel an inexplicable chill. Just before dawn, Rick hears the eerie sobs of an unseen woman, a phenomenon that Pamela investigates whilst awaiting her brother's return with their Irish housekeeper, Lizzie Flynn (Barbara Everest). The superstitious Lizzie notices a peculiar draft on the stairs. Rick and Pamela must face the obvious: Windward House is haunted.
When Stella comes to Windward for dinner, she senses a spirit. Rather than fearing it, she associates the calming presence with her mother. Also, the strong scent of mimosa is that of her mother's favorite perfume. Suddenly she dashes out towards the very cliff from which her mother Mary fell to her death seventeen years earlier. Rick catches her just before she reaches the edge. Stella professes to have no recollection of the near-fatal incident.
The Fitzgeralds and the town physician, Dr. Scott (Alan Napier), investigate. They learn that Stella's father, a painter, had had an affair with his model, a Spanish gypsy named Carmel. Stella's mother, Mary Meredith, from all accounts a beautiful and virtuous woman, found out and took Carmel to Paris, leaving her there. Subsequently Carmel returned to England, stole the infant Stella from Windward, and, during a confrontation, flung Mary Meredith off the cliff to her death. Shortly afterward, Carmel became ill and died.
Rick tries to dissuade Stella from her dangerous attraction to Windward by staging a sÃ©ance to convey the "message" that her mother wants her to stay away. However, the ghost takes over and communicates that it is guarding Stella. Stella becomes possessed by the spirit and begins muttering in Spanish. Distressed by Stella's renewed involvement with Windward, Beech sends Stella to a sanitorium run by Miss Holloway (Cornelia Otis Skinner), Mary's friend and confidante. The Fitzgeralds visit and question Holloway, unaware that Stella is confined there. Holloway explains that after Mary's death, she took care of Carmel, who had contracted pneumonia and eventually died of it. Looking through the records of the previous village physician, Dr. Scott discovers that Holloway may have hastened Carmel's death.
The doctor is called away to care for an ailing Beech, who tells him that Stella is at the sanitorium. Rick, Pam, and Scott telephone Miss Holloway to inform her that they are on their way. Holloway deceives Stella, saying that the Fitzgeralds have invited her to live with them. Stella happily takes the train home. Holloway tells the would-be rescuers that Stella is on her way to Windward House. There Stella finds only her grandfather in the studio. He begs Stella with his last strength to get out, but she remains at his side. When a ghost manifests, the commander succumbs to a heart attack. Stella welcomes the ghost, believing it to be her mother, but the apparition frightens her, and she flees towards the cliff.
Rick and Dr. Scott get there just in time to pull Stella from the crumbling cliff to safety. Back inside, the group is drawn again to the physician's journal, which the friendly spirit has opened to a certain page. They discover that Carmel gave birth to a child in Paris, where Stella herself was born. The truth becomes clear: Carmel is Stella's mother. Stella's realization of her true parentage frees Carmel's spirit to leave Windward. Something evil, though, has remained. After sending everyone away, Rick confronts the spirit of Mary Meredith, telling her that they are no longer afraid of her and that she has no power over them any longer. Defeated, Mary's spirit departs.

Question: The doctor is told that who is in the sanitarium?
Answer: Stella.