The science-fictional universe in question in this small wonder of a novel is Minor Universe 31 (MU31), where a sad-sack time-travel-machine repairman ekes out a living with his dog, who's mostly hypothetical, and a computer with a sexy feminine AI interface that he has a crush on. In Charles Yu's world, human beings use time machines mostly to go back and eavesdrop on their screwed-up lives, reliving key moments, bad decisions and missed opportunities in the mistaken belief that they can change them. They can't. "I have job security," the repairman explains, "because what the customer wants, when you get right down to it, is to relive his very worst moment, over and over again."