The film begins with Sybil as a young woman working as a substitute teacher in New York City, she experience blackouts, memory lapses and disturbing imagery (memories). Sibyl decides to seek medical intervention after putting her hand through a window in her apartment while experiencing one of her episodes. It is here that she meets Dr. Wilbur who is called in for a neurological consult. Dr. Wilbur takes an interest in Sybil and finds out from her that she has been having these episodes for as long as she can remember. Dr. Wilbur then explains to Sybil that she is suffering from a kind of hysteria that causes her to black out under conditions of fear. Dr. Wilber then offers to take Sybil on as a patient. The next scene shows Sybil discussing the possibility of seeking medical care for her moodiness with her father who is very skeptical, and mentions how their church feels about the practitioners of the mind. Sybil then loses her job. She phones Dr. Wilbur at 3 a.m., as Vicky, as Sybil is contemplating jumping out of a hotel room window several stories up. There is shown a flashback to Sybils childhood to a traumatic scene when she had her tonsils removed. It is in this scene that Dr. Wilbur discovers the multiple personalities that reside within Sybil. Dr. Wilbur is shown speaking with another psychotherapist, apparently her mentor, who warns her not to fall in love with the patients illness, as this may contribute to more suggestible symptoms. As the film progresses, many of Sybils different personalities are showcased as telling things about one another and Sybil, telling things that Sybil cannot discuss on her own. After dissecting the stories that each personality is telling to Dr. Wilbur, Dr. Wilbur decides that it was some sort of traumatic childhood event that has caused Sybils personality to shatter. By this time Sybil is becoming friends with a male neighbor, Richard a young widower with a small son, who seems to be very taken with her. Sybil continues to undergo therapy with Dr. Wilbur and continues to exhibit her personalities, changing personalities at times of stress. Dr. Wilbur attempts to explain to Sybil that she was traumatized during childhood causing her personality to split into many childlike personalities; Sybil does not want to accept this explanation for what is happening to her. Upon hearing herself imitate her own mother, she disassociates into a baby. Sybil continues to see Richard and their relationship progresses, on Christmas Eve Sybil agrees to let Richard stay the night, he agrees that he will only hold her close. Sybil has a nightmare in which she is being pursued by a decapitated cat and ends up climbing to the top of her bookcase, waking Richard. It is at this time she discloses to Richard that she is seeing a Dr. Wilbur. He guesses that Dr. Wilbur is a psychiatrist and runs to the nearest payphone and calls information to get Dr. Wilburs phone number. He calls Dr. Wilbur, who is at a dinner party; Dr. Wilbur breaches patient/client confidentiality and discloses to Richard that Sybil suffers from multiple personality. Sybil then climbs to the roof of the building and Richard asks Dr. Wilbur to come right away. Upon her arrival, Dr. Wilbur finds Richard holding Sybil who had just made an attempt to jump off the roof of the building. While under the effects of the medication Sybil discloses that she does not want to see Richard anymore until she gets herself together, as she is falling in love with him. Richard hears this and moves out of his apartment.
Fast forward a few months, Sybil is in Dr. Wilburs office, she denies that she has multiple personality, she states that she had been putting on an act all along and that her mother never abused her. Dr. Wilbur is not sure she believes Sybil, but thinks this may be the beginning of her healing in that the personalities are coming together as one. Dr. Wilbur decides to take an investigatory trip to Chicago to speak to Sibyls father, and then to Sybils hometown in Wisconsin and visits Sibyls pediatrician. The pediatrician remembers Sybil well, and her nervous mother, and shares his records with Dr. Wilbur. The medical records are indicative of the abuse that Sibyl had described. The film ends with Dr. Wilber taking Sybil on a picnic in a secluded area, encouraging her to paint, when she cannot paint, Dr. Wilbur performs hypnosis on Sybil, it is then that Sybil meets the rest of her personalities and they become one.