Watching Rivals, I found myself alternating between moments of intense desire to see what happens next, on the one hand, and a mix of boredom and annoyance on the other.
Widowed mother Christine (Joan Hackett) is providing inconsistent discipline to her 10-year-old prodigy son Jamie (14 y.o. Scott Jacoby), who resents her dating anyone. Jamie's attachment to his mother may be partly sexual, or he may just be spoiled, not wanting anyone to have what he can't have, as shown when he drives a wedge between his baby-sitter and her boyfriend. When Christine remarries, to the much more happy-go-lucky (and annoying) Peter (Robert Klein), Jamie slowly succumbs to psychopathy, and events go poorly for everyone.
The story line is very believable, other than portraying prodigal mental ability as linked with precocious sexual interest and development. (This may just reflect the state of 1970s science). Hackett and Klein provide their usual top professional performances. And young Scott Jacoby does a terrific job playing a much younger boy, even when he acts out as more childish to manipulate the adults. Some viewers may complain about the make-out scene between Mary and 10-year-old character Jamie, although the actual content is relatively mild (limited contact, primarily implied or simulated) and the actor (Jacoby) is actually 14. Jeanne Tanzy Williams comes across very credibly as Mary, a teen girl facing the always difficult sexual quandary of When Will I, and With Whom.
Another director might have made an almost-great film out of this. Unfortunately, periods of not much happening, intended to show relationships having time to grow, last way too long and don't provide side benefits such as character development. Even more annoying were several frantic video montages that were too disjointed to convey plot points coherently and too rapid to place in context. And the soundtrack, as many reviewers have noted, was bizarre, ranging from saccharine to pseudo-funkadelic; it rarely adds and often detracts from the experience. Still, the acting and story line make this film worth watching once.