Troubled Columbia University art student Sybil Dorsett is referred to psychiatrist Cornelia Wilbur by Dr. Atcheson, a male colleague who believes the young woman is suffering from female hysteria. As her treatment progresses, Sybil confesses she frequently experiences blackouts and cannot account for large blocks of time. Wilbur helps her recall a childhood in which she suffered physical, emotional, and sexual abuse at the hands of her disturbed mother Hattie, and eventually 16 different identities varying in age and personal traits begin to emerge. Chief among them is Victoria, a French woman who explains to Dr. Wilbur how she shepherds the various parts of Sybil’s whole. Frustrating the therapist are objections raised by her associates, who suspect she has influenced her patient into creating her other selves, and Sybil’s father, who refuses to admit his late wife was anything other than a loving mother.
Although she had promised never to hypnotize Sybil, later into the treatment, Dr. Wilbur takes her patient to her home by a lake and hypnotizes her into having all 16 personalities be the same age as she and become just aspects of Sybil. By nightfall Sybil claims she feels different and then emotionally declares her hatred towards her mother.
The last part of the movie tells of the history of the real woman who was known by the pseudonym of Sybil Dorsett.