Josie and Rick are in love in a fraught, teenage way. The obstacle in their relationship, however, is that Josie has been "lifted," while Rick has not. "Lifting" in this novel is a form of genetic enhancement that allows a person to excel academically. It means that Josie will almost certainly be able to get into a good university and have a well-paying career, with many opportunities in life. However, the lifting procedure comes with the risk of death: Josie's sister has already died, and Josie herself is unhealthy, even to the point of becoming deathly ill, because of the lifting.
Rick's mother has refused to take the risk of lifting him, so Rick's options in life are limited from the start. He may get into a good university, but it is very much a long shot. He has to prepare himself for a lower-income, more working-class kind of life. However, he is at no special risk of dying young.
The tension arises because even as a teenager, Rick is not accepted into Josie's social class of lifted peers, as is made clear when Josie's friends (or peers—they are not really friends) are condescending to Rick at Josie's party. Rick reacts badly, as one might expect, to being treated as an inferior. Rick and Josie know their difference means they will probably go their separate ways in life, but Klara comes to understand that they will always carry with them the love they once felt for each other.