To forestall the theft of his most promising work, computer whiz J.J. borrows his mentor's closely guarded time-machine and travels into the past, where he tries to foil the thief without destroying his own future. At 16, J.J. is a successful inventor, for two years a graduate student of Professor Poplov's (an authority on holography) and the complacent boyfriend of lovely Polly Ambruster. Telling his own story, J.J. also reveals himself as a garrulous nerd. But when he discovers that Max Sharp is horning in on Polly, as well as coming up with electronic moneymakers remarkably like his, he reasons his way into Poplov's ""Pandora's Box"" and, miscalculating, ends up four years back. He meets himself (Jacko) at age 12, is appalled (reciprocally) at his own pomposity, and finds computer-tapping well under way. He and Jacko go back another four years (now they are three), and there's some frantic negotiations with Poplov, who wonders whether the trouble-making time machine should ever be built--a logic-spinning riddle. Though characterization is minimal, Curry makes the most of her intriguing idea, getting her three main characters back to the present as one plausibly integrated personality in spite of the paradoxes she raises. Comedy grows naturally out of the situation of self confronting self; there's a satisfying degree of suspense and some nifty twists and insights. Good fun.