The best fly caster in America,"" feeling twinges of age, bids for immortality by constructing the ultimate trout stream and stocking it with a single superlative specimen--his record-catch-to-be. A local boy, a lowly bait fisherman, sees his virtuoso casting. . . and sets out to be as good or better. Add in the fact that Major Quillaine is childless, and young Plummey Pittock is fatherless. Add in also old Mr. Sluter, the master rod-maker, now blind, who teaches Plummey fly fishing technique and the ways of trout, who helps him build first a practice rod and then a perfect example (out of bamboo the Major covets). Hyde patently aims to be writing a folk legend of a magnificent obsession, an old champion and a young challenger; and he hasn't the consistent, robust style to carry it off. (There are lapses into internal monologue. There's some mawkish love-interest: the object of Plummey's tongue-tied affections turns out to be avid about fly-fishing too.) But Hyde does make palpable the Major's vision of a trout stream replicating the great trout-fishing experiences of a lifetime (""Here, on one pool, he was back at the Laughing Whitefish,"" ""Now came a series of rock runs and pools from the Lamoille""): a human creation, moreover, that justifies itself as a nature preserve (""Water, cover, and privacy had made a difference. . . wildlife comes best to those properties where it is loved""). As for Plummey's challenge to the Major, suffice to say that he proves himself a worthy heir without robbing the Major of his triumph. It's a fail-safe plot, if a hoary one--and the trout-fishing excitement is infectious. JLG.