Billy Lenz, a boy born with severe jaundice, is constantly abused by his mother, Constance. With the help of her lover, she murders Billy's father Frank on Christmas Eve 1975 and buries his body in the house's crawlspace. When Billy witnessed her scheme, she imprisons him in the attic. In 1982, Billy's mother impregnates herself from her own son in the attic due to her boyfriend's impotence to conceive another child. Nine months later, Constance gives birth to their daughter Agnes and uses the occasion of Agnes' birth to further reject Billy. On Christmas Day 1991, Billy escapes from the attic and disfigures eight-year-old Agnes by gouging out her eye. He then brutally murders his mother and her lover. He is caught by police eating cookies made out of his mother's flesh and is sent to a mental asylum while Agnes is taken to a local orphanage.