Everyone knows the almost magical power of the computer's "undo" command that can erase any mistake. Well, thanks to a device called the "under", middle-school student Gib Finney can do the same thing in real life. At first he thinks this is better than winning the lottery: he'll never fail another test, never lose another bet, never strike out in a ball-game again! But after a terrible accident, Gib must decide which events in a two-day period he needs to change to prevent disaster. Was the key when he and his friend Ash saw signs advertising a carnival? Or was it when he argued with Rainy Rogner about their experiment? Or perhaps when he shot a spitball at the math teacher? Gib soon learns that some things are more worthy of "inning" than others, and that some events may be impossible to change at all.