A fifth adventure featuring Aristotle ``Soc'' Socarides, Cape Cod p.i., fisherman, diver, and general knockabout (Feeding Frenzy, 1993, etc.), is that common entertainment-fiction combination of the lethal and fairly harmless. Here, Soc is tapped by an old Vietnam buddy to come to the aid of Native American activist Joe Quint, a charismatic rouser of rabble who's possibly been framed for the murder of a tourist theme-village night watchman. Quint so much doesn't want to talk to Soc that he breaks jail and goes into deep hiding. But evidence found during an underwater archaeological expedition, visits by friendly and hostile Indians, and the desire to protect pretty women, among them Quint's idealistic lover Patty, pull Soc into conflict with corrupt gambling interests. Hyperbolic Quint had once advocated the pulverization of Plymouth Rock; let's just say that, in bomb-happy America, he should have been careful what he wished for. Kemprecos can be an appealing enough writer (``Peter was a likeable guy. It was going to be tough pushing the down button on his elevator''), but without the beach-bum existentialism of John D. MacDonald and the bankable funniness of Robert B. Parker, Soc seems merely (and inconsistently) a third-string substitute. Many readers, of course, will find that enough.