This post contains lots of spoilers!!
This book is told in split first person. Day is a street kid who's run away from home after failing his assessment test. He should have been sent to a work camp but instead escapes to the streets. June is a prodigy - the only one to get a perfect score on her test. She is destined to be a Republic soldier.
Day finds out his family's home has been marked as having the plague. He attempts to find plague cures (which are very expensive). He's unsuccessful and ends up getting a fight with June's brother Metias. Day injures him and escapes. Day's friend and tag-along Tess helps stitch him up.
Metias ends up dead and everyone is convinced Day is behind it. June is pulled out of school early and put into service under Cmdr Jamison. Her mission is to find Day. She goes under cover as a street kid and quickly finds and befriends Tess and Day, although she doesn't realize who he is. And, of course, they end up kinda falling for each other. They kiss and shortly after, June pieces things together and realizes he's Day.
She executes her plan to turn him in. In the ensuing scuffle, his mother is shot and killed by Metias' friend/collegue Thomas. June is horrified, but sticks with the mission. Day and his two brothers (one who has the plague) are taken into custody. June is in charge of interrogating him.
Eventually June starts to get curious. She looks up Day's test score to find out that even though he was marked as failing, all of his answers are the same as hers - he should have gotten a perfect score too. Then she finds Metias' journal/blog which shows that he was also getting suspicious of the government as well. He uncovered that their parents didn't die in a car crash and were instead killed by the government because they started asking questions about the plague. Essentially the plague is used as population control and to keep everyone in line since the government provides the cure. She also looks up the files surrounding Metias' death and realizes that Day wasn't the one who murdered him. Day injured him, but he was murdered by Thomas, who has recently been trying to get June to get romantically involved with him.
She decides to try to break Day out before his execution. She recruits the Patriots (essentially rebels) to help and discovers that Tess has teamed up with them. They agree to help for a good sum of money. Adventure and danger ensue but the bottom line is that they successfully escape, but Day's older brother Jon sacrifices his life to aid in their escape. June and Day get as far away as possible. They plan to head to Las Vegas, where they believe the Patriots and Tess are, to make sure that she's safe, and then attempt to rescue his younger brother Eden.