A science-fiction--yes, science-fiction--novel from the master of soap. And one with a MESSAGE, too, just like the sf of yore--the clichs of which Sheldon shamelessly recycles as he ham-handedly depicts an earth under threat of invasion by aliens ticked off at- -what else?--our destruction of the environment. US Navy Commander Robert Bellamy--Sheldon's first male lead in many years--is assigned by NSA to locate the 11 people on a Swiss bus who saw the crash of a ``weather balloon.'' It takes only a chat with the bus driver for Bellamy to learn that the ``weather balloon'' was really a downed UFO containing two alien bodies. It takes talks with all the witnesses, however--Yank, Soviet, Hungarian, etc., each tracked down in the novel's repetitive first two-thirds with minimal sleuthing but maximal scenery-stuffing--for him to learn that each is killed right after talking to him: ``It was an international conspiracy, and he was in the middle of it.'' And so are: the aliens (``a form of vegetable life'' whose eyes ``resembled Ping-Pong balls'') circling earth in their mother ship, waiting to see whether world leaders will respond to their secret plea to halt pollution; the missing third occupant of the UFO, dying for lack of pristine water; and the international cabal, led by ``Janus,'' that's killed the witnesses with the intent of fighting the aliens and continuing earth's exploitation. In the livelier last third, Bellamy, resorting to clever spy-tricks and help from a winsome whore, runs from Janus--whose identity you'll spot chapters away--while plotting his downfall. The fitful action climaxes in an Alpine showdown, with the celestial calvary soaring in for the rescue. Inane as sf (and seemingly cribbed in part from sources ranging from John Campbell's ``Who's Out There?'' to Whitley Strieber's Communion); mediocre as a thriller, even Sheldon-style; but fascinating as one top author's earnest if inept effort (backed by a polemical