From screenwriter Shapiro, a script-length short novel that (opportunistically?) resembles last year's blockbuster sf comedy Back to the Future. In 1985, David Russell bears a perpetually heavy heart for his wonderful older brother Christopher, a captain killed in Vietnam. If only Lee Harvey Oswald hadn't shot President Kennedy, who was withdrawing US troops from Vietnam, Lyndon Johnson wouldn't have become president and escalated the war. David has fallen for Laura, a brilliant host for educational TV in Dallas, who understands his long-lived grief for Christopher. Then David meets Nobel Prize-winning physicist Dr. Hendrik Koopman, a chubby Dutch cherub, who has invented a time machine. If David can persuade Dr. Koopman to send him back to the roof of the Texas School Depository on November 22, 1963, he can stop Oswald's fatal bullets and avert the Vietnam war. A terrific idea, Dr. Koopman agrees, but when Laura finds out what they're up to, she's less enthusiastic. Once back into 1963, David Finds he can't get down from the roof in time to stop Oswald (the roof door's stuck), and through a contretemps the police seek him for the killing, while Oswald is eventually a decorated hero with a park named after him. When--in the future--Laura and Dr. Koopman find out what's happening to David (they are following this reversal of events by microfiche copies of 1963 Dallas newspapers), they send Laura back to the day before the assassination (David hasn't even arrived in the past yet) to alert the FBI--but she's hit by a car, and again the assassination still takes place. A third possibility lies before Dr. Koopman, mid he makes a momentous decision. Lightweight stuff indeed. Shapiro hacks away serviceably, but his only moments of high imagination are when the characters' minds are invaded by waves bearing the burden of time revised. Wait for the movie, with its period music, costumes and set decoration. The last scenes could surge with a sentimental power that the book only sketches in.