Millicent, a prudish
brunette, is consumed by hatred for her twin sister Therese, a seductive and mean-spirited
blonde. Millicent confronts Thomas, Therese's lover, and reveals Therese told her all about an unspecified immoral event that happened during Thomas and Therese's sexual relationship. She explains that Therese does not care for Thomas and is only trying to corrupt him with her evil. Millicent confides in her friend and family therapist, Dr. Ramsey, that her sister
engaged in sex with their father, poisoned their mother, and is holding her captive inside the family mansion, while gloating to Millicent about her deeds. Ramsey visits the mansion to speak with Therese, who comes on to Ramsey and furiously throws him out of the house when he refuses her advances.
Millicent writes a letter to Dr. Ramsey, explaining that she has determined that Therese is evil and that she will stop her even if that means losing her own life, and plans to use a
voodoo doll to kill her. When Dr. Ramsey enters the house, he finds Therese dead on her bedroom floor with the doll next to her and no apparent cause of death. Dr. Ramsey removes Therese's blonde wig to reveal Millicent's dark hair, and also reveals that "Therese" and "Millicent" are the same person; Therese suffered from
multiple personality disorder brought on by her having slept with her father and subsequently killed her mother. "Millicent" was an alternative personality with a repressed sexuality to cope with the horror of her actions. The recent death of her father unhinged her further, and the "murder" was actually suicide.