Everyone knows that a computer's "undo" command can erase a mistake. Gib Finney has been given a device that allows him to do the same thing - in real life. At first, the possibilities seem endless. Flunk a test; take it over again. Keep swinging at the same pitch until you finally hit the winning home run. But when his younger sister is gravely injured in a traffic accident for which he feels responsible, Gib has to figure out which events in a two-day period should be changed in order to ensure that the accident never takes place. Did it all begin when Gib and his friend Ash set out for the carnival? Or when he argued with Rainy Frogner about the salt in their science experiment? Or when he shot the spitball at his math teacher? Gib finds himself "correcting" far more than he intended to, and the consequences quickly become impossible to predict.