Like most citizens of the repressive, ultrareligious New Kingdom, sixteen year old Mercy tries hard to fit in and be happy. She doesn't want to be abnormal like her crazy brother Keenan, who talks about escaping to what he believes will be a better life on the outside. Mercy knows what every citizen knows -- outside the New Kingdom, there is nothing but a war-ravaged wasteland. She does her best to silence her brother, fearing that he'll be sent away to Renewal again, or worse -- that he'll infect her with his discontent.
But when Mercy makes friends with rich and privileged fellow student Violet, she finds that she can no longer deny her own dreams (what dreams? you just told us she wants to fit in, not rebel) The Guardians have arranged for Mercy to marry a young man she barely knows. But as their friendship grows, Mercy comes to realize that she is in love with Violet. Despite her fear that her friend will reject her and report her to the authorities, she finds the courage to confess her forbidden feelings, and is astonished when Violet reciprocates.
Mercy now realizes her brother is right. There is no chance for happiness inside the New Kingdom. Knowing it will be difficult and dangerous, and that they will face severe punishment if they are caught, the three young people conspire to flee their home and venture into the Outerlands, where no citizen has ever set foot.
I'd be a lot more interested in this if it had something unusual in it. Like the Guardians want her to marry a woman but she secretly likes men.