Patricia Carroll (Stefanie Powers) is an American woman who travels to London to marry her boyfriend, Alan Glentower (Maurice Kaufmann). While there, Patricia stops by to visit Mrs. Trefoile (Tallulah Bankhead), the mother of her deceased ex-fiancé, intending to pay her respects. Upon arriving, however, Patricia discovers that Mrs. Trefoile's grief for her son has transformed her into a sociopath. When Mrs. Trefoile begins holding Patricia prisoner, she must find a way to escape.