The spoiled Long Island heiress, Judith Traherne (Bette Davis) is suffering from severe headaches and visual disturbances, which she tries to ignore in pursuit of wild parties and frenetic horse-back riding. Her friend and secretary, Ann (Geraldine Fitzgerald), and the old family doctor bring her to Dr. Frederick Steele (George Brent). Steele has just sold his neurosurgical practice and is about to catch a train to Vermont where he will devote himself to fulltime research on malignant brain tumors. He delays his departure to operate successfully on Judith's glioma.

He and his patient fall in love. But Steele learns that her disease will recur in a predictable manner: blindness followed by painless death. He and Ann conspire to hide the prognosis from Judith. A wedding is planned and the move to Vermont. But Judith uncovers the secret and flies into a rage at the treachery, breaking off the engagement.

She tries to drown her sorrow of impending doom in drink, a frivolous dalliance with a drunken suitor (Ronald Reagan), and a more serious dalliance with her horse-trainer, Michael (Humphrey Bogart). On the verge of sin with Michael, she realizes that her only hope lies in life itself, marriage and the house in Vermont. Steele conducts his laboratory research in a back shed and the couple carry on as if her death sentence did not exist.

Living a lie provides their few months of happiness, their "Dark Victory," Judith says, over the cruel promise of her death. Just as Steele is invited to New York to present his research, her vision begins to cloud and fade. She tells Ann, but keeps the news from her husband. He leaves the now blind Judith and her friend in the garden planting bulbs that will bloom in spring. She sends Ann away too, and lies down to face her romantic (but painless) end alone.