Frank Carveth (Art Hindle) has a wife, Nola (Samantha Eggar) who is mentally ill (the exact nature of her "breakdown" is never made clear, but it is implied that she was abused as a child). Nola is an in-patient at the Somafree Institute of Psychoplasmics run by Dr Hal Raglan (Oliver Reed). Raglan treats patients by engaging in intense role play encounters in which he takes the part either of parent or child to the patient.

The result of his approach is the somatization of emotional problems, his logic apparently being that allowing psychopathology to manifest in the (medically treatable) body liberates the less accessible psyche from illness or harmful emotions. So, for instance, a man with unresolved anger against his father develops sores all over his body during therapy. Their healing enacts his catharsis.

There are problems, however: another patient attributes his terminal cancer to Raglan's therapy, saying "psychoplasmics . . . encouraged my body to revolt against me and it did." Most terrifying of all is Nola's rage. It expresses itself in the form of strange buds that appear on her abdomen. These develop into external wombs, or amniotic sacs, from which she keeps giving birth to deformed and malevolent children.

These children, "the brood," literally enact her rage, escaping from Somafree to attack and kill anyone who is the object of Nola's anger, including both her parents and, eventually, Dr. Raglan himself. When the brood turns on Candy, Frank and Nola's actual daughter, Frank strangles his wife, and her evil offspring die with her.