Put-upon teen-age hero wins heart of spoiled rich girl while saving planet from comic-evil alien invaders. Yep (Seademons, 1977, etc.) has been trying to break out of the juvenile/YA mold for some time, but on this and past evidence he's never going to make it. On planet Carefree, young Piper Kincaid and his father ("Dad") run Monster Makers, Inc., a genetic engineering concern that produces miniature dinosaurs, mammoths, and other specially-created creatures for fun and profit; recently, though, their business has been dogged by ill luck. When a four-foot-high godzilla escapes and rampages through a nearby swank resort, Piper goes to recapture the creature and meets Shandi, daughter of a super-rich industrialist. Together they discover that what Piper has assumed to be accidents were actually calculated acts of sabotage. The alien Xylk, you see, together with their warrior-slaves, the Rell, have established an underseas base as a prelude to invasion; too, some Xylk have disguised themselves as local life-forms, and the Kincaids have the only analyzer capable of uncovering the deception. Various complications ensue before Piper thwarts the Xylk almost single-handedly. The teen-age leads are believable enough, though Dad is a cypher, and their rocky romance is well-handled: sprightly and often amusing, then, but one-dimensional--so put this one on the YA shelf.