John Mallory's 4-year-old son had been kidnapped. Fifteen years later a photograph came through the mail of a teenage boy who could have been the missing Stevey. Handily enough, lawyer Mallory's adopted daughter was dating an ex- F.B.I. man and they team up to track down and save the toddler turned tough. It all turned out to be the longest revenge plot on record, solved as it was written -- with a very mechanical efficiency and swathed in melodrama.