First things first: if you plan to watch this 1993 telefilm, suspend your disbelief immediately.
Now, onto the film. An all-American clean-cut kid murders his adoptive parents and commits suicide. A gorgeous crack reporter (Welch) is on the case, writing a book about kids who kills their parents and digging into the dirt of this baffling case. After two brief "moral issue" scenes (one questioning, for half a second, the role of the media in perpetuating violence, the other questioning the role of genetics in shaping future society), her investigations lead her to a shocking discovery: not only was the kid's murder of his parents caused by a genetic deficiency, but he also had a twin sister and she, of course, could blow at any time and murder her adoptive parents. Looking quite lovely throughout, Welch is on the case, cool and glam as she hunts down this genetically pre-disposed murderous teen.
Cut to Suburban USA, where two adopted teen girls live across the road from each other. One has the sane Alley Mills from "The Wonder Years" as her mother; the other suffers the far more fascinating Joan Van Ark from "Knots Landing" (expanding admirably on her histrionics from telefilm "Always Remember I Love You" a few years earlier, where she also played the highly-strung adoptive mother of a troubled teen).
This film is quite implausible and unbelievable, but it is diverting enough if you have nothing better to do - and it is watchable for Van Ark's scenery-chewing performance alone (although the rest of the cast do their share of scenery-chewing too!)