Jason and Marceline continues the stories of Jason Herkimer and Marceline McAllister, who appeared in Spinelli's earlier novel, Space Station Seventh Grade. Jason and Marceline begins with Jason and Marceline walking to school together for their first day in the ninth grade. It ends with them attending their ninth-grade prom.
Jason is concerned with being accepted by and liked by his friends. Marceline is more self-contained and inner-directed; she is more concerned with doing what she feels to be right, as well as with her own personal interests. Even though these differing attitudes cause some problems in their relationship, they begin and end the book as boyfriend and girlfriend.
In the course of their year in the ninth grade, they face all kinds of pressure. Jason feels pressured by his peers into sexual experimentation. Marceline, however, demands that the two of them go at their own pace. At one point, they even break up over problems of this sort. Then, Jason finds a partner, Heather Newsome, who is willing to do some of the things Marceline refuses to do: give him hickeys and wear his ankle bracelet. Heather says that she loves Jason and asks him to go to the prom with her. He accepts. Eleven days later, Jason discovers Heather kissing Hughie Muntz. Jason and Marceline get together again after Jason, remembering the Heimlich maneuver he learned in health class, saves the life of a seventh grader who is teased mercilessly by the ninth-grade boys.
Jason and Marceline provides some insights into the kinds of problems ninth graders face. Mostly, however, it focuses on more humorous aspects of growing up and of waking sexuality.