Delancey Sullivan knows for sure that there is more than one universe. Many more. Because every time someone makes a choice, a new, parallel world spins off the existing one. Eating breakfast or skipping it, turning left instead of right, sneaking out instead of staying in bed - all of these choices create an alternate universe in which an echo self takes the road not travelled. Del knows all of this because she's a Walker, someone who can navigate between the worlds, and whose job is to keep the dimensions in harmony. But Del's decisions have consequences too. And when she secretly starts to investigate other dissonant worlds she's intrigued by the echo versions of Simon Lane, a guy who won't give her the time of day in the main world, but whose alternate selves are uniquely interested. But falling for Simon draws Del closer to a truth that the Council of Walkers is trying to hide - a secret that threatens the fate of the entire multiverse.