She writes too seldom, does Ngaio Marsh, for her socialite detective, Roderick Alleyn, has a deft touch, and combines savoir faire with keen analytical abilities, quite in the Scotland Yard tradition. The crime, this time, is set on the bank of a trout stream near the village of Swevenings. The main families are definitely county, and it seems incredible that motive, opportunity and weapons at hand all seem to involve the Lacklanders, whose escutcheon has been unblemished. Alleyn not only. uncovers the flaw, but manipulates a local feud over a giant trout -- and an old scandal- and a new romance- and various other byways, to the end that the one unlikable person of the lot turns out, most conveniently, to be the murderer.