Young Roxanne, having spent most of her life in a mental institution, is released after having been declared cured. She goes to live with her sister Michelle--but Michelle soon begins to notice that there is something not quite "right" about Roxanne. One thing she does is make sure that Michelle's husband Tony constantly gets an eyeful of her shapely young body in a thong bikini. Another habit she's developed is performing satanic rituals in the nude at night in the back yard. This and other odd behaviors leads Michelle to believe that her beautiful young sister might be possessed by a succubus--a demon that gets its power from seducing and then killing young, virile men.