And Along Came Ginger
While this is definitely a sequel to the original Taboo and its two sequels, and was written and directed by the same husband and wife team, this film isn't another installment in the story of the relationships (in many senses of the word) of Barbara Scott (Kay Parker, who appears in this film only in archive footage.) That said, Honey Wilder and Kevin James are back as Joyce and Junior McBride, from the second installment. (Oddly, James skipped the third film, even though Wilder's character in that one was incestuously involved with a character supposed to be her son.) Joyce is apparently sort of regretful of her relationship with her son, and is in group therapy moderated by Dr. Jeremy Lodge (Jamie Gilis).
And this film becomes Dr. Lodge's story, or to be accurate, it's the story of him and his two daughters -- Naomi (Karen Summers) and Robin (Ginger Lynn, pretty close to the start of her volcanic career in adult entertainment.) Except that it comes out that Naomi isn't his daughter, but the child of an affair that the Doctor's wife has been having with his brother (John Leslie). The Doctor finds this out at the worst possible time, just after his daughters have been expelled from a private school after being caught in a compromising situation. (More on that in a second.) He kicks his wife out, along with "bad girl" Naomi, which leaves him in a strangely vulnerable situation with "good girl" Robin.
Because, and this is the film's strength that it never really exploits to the fullest, Dr. Lodge has a blind spot where Robin is concerned, and we are shown that she's not a "good girl". Where he believes that Robin was tricked or manipulated into a sexual encounter with her sister's boyfriend (and her sister, in the series' first instance of incestuous lesbianism) the audience is allowed to see that she was a willing participant (though she tries to keep that a secret even from Naomi and the guy, letting them think that they're taking advantage of her while she's asleep.) Later, she listens in on the therapy sessions, and seduces Junior McBride away from his mother (who isn't really making a serious effort to change her ways, apparently.) And of course, this ultimately leads to her luring her father into her own bed, as he's become desperately lonely without his wife. Meanwhile, Naomi is engaging in sexual experimentation of her own, including sleeping with Uncle Billy (who's really her father, remember) and other guys. Yet she's open and forthright about her desires, instead of tricking her partners, as Robin does.
What could have made this film excellent, and further exploited this dichotomy between perception and reality, is if it were ultimately revealed that Naomi was Dr. Lodge's biological daughter, and Robin was the brother's child. Instead, what we get is a mildly disturbing conclusion where Dr. Lodge and and Robin murmur a strange parody of wedding vows to each other at the end of their sexual encounter, promising forever ... and as I've pointed out in reviews of other films in this series, adult films can't be about monogamy. And the next film in the series would illustrate that further.