The first two adventures for Sam Holt--TV actor, amateur detective--had a certain panache, blending L.A. glitz with a wry N.Y. sophistication reminiscent of Rex Stout. This time, however, stuck on a Central American isle in a blatant, feeble imitation of Christie's And Then There Were None, Holt's distinctive charm is pretty much lost in tile surrounding artificiality and silliness. Four actors famous for playing detectives, you see, have been brought to an island castle to film a TV commercial. In addition to Holt (TV's ""Packard""), there's Harriet Fitzgerald (Miss Marple), Clement Hasbrouck (Sherlock Holmes), and Fred Li (Charlie Chan). Also on the premises: Holt's girlfriend Bly, Hasbrouck's cozy wife, Li's lush girlfriend (a black singer), and Fitzgerald's lesbian companion Daphne. Plus nervous director Jack French and the two TV producers who own the castle (the former property of a drug-king). Before any filming can happen, however, the showfolk are cut off from the mainland by a storm, of course. Then Daphne is found dead--apparently a suicide, really murdered. Next: the disappearance of Jack French, the near-fatal poisoning of Mrs. Hasbrouck, the discovery of two more corpses, etc.--as the actor-sleuths do lots of searching and talking, suspecting each other, dredging up possible motives for the killings. . .until the ludicrous truth is revealed in a stagy finale. Sheer nonsense, delivered with show-biz chatter and a modicum of slickness--but not enough to keep this from being a big letdown to followers of Holt's promising new series.