In S.E. Hinton's follow-up to the classic of teen fiction The Outsiders, she writes a more mature story called That Was Then, This Is Now. This novel is about how teen friendship evolves. The lead character is Bryon, and his best friend is Mark. They are close friends with a strong bond stretching back to childhood. Mark loses both parents and lives in Bryon's home with him and his mom.

During most of the story, Bryon's mom is in the hospital, so the teens are raising themselves at the mere age of sixteen. The plot involves pool hustling, Bryon meeting his first serious girlfriend (Cathy), and an older friend (Charlie) defending the two boys in a bar fight, in which Charlie is shot and killed.

Charlie's death affects Bryon and Mark differently. For Bryon, it causes him to get serious, get a job, and start dating Cathy seriously. Mark and Cathy don't get along, and Bryon feels caught between them. One night, Mark and Bryon finally get to spend a night alone, in which a series of events leads to Bryon trying to locate Cathy's younger brother, M&M, and finding him on a bad LSD trip.

In the course of trying to locate M&M, Bryon discovers drugs under Mark's bed, realizes he's been dealing, and calls the cops on him. Mark is arrested, and Bryon testifies at the trial. He later visits Mark in prison, but the latter now hates his guts.

Mark learns from Bryon that decisions about right and wrong aren't simple, as they were in childhood. Actions have real-world consequences, and you can't go back in time. Bryon learns from Mark that friendship is valuable, but it can't keep a person from doing what is right.

As for changing, Bryon learns that Mark's jealousy of Cathy, his unwillingness to forgive, and his decision to deal drugs mean that Bryon is fundamentally different from Mark. He learns from Mark what his true values are and how to stand up for them.

Bryon learned from Mark that one's actions can negatively affect other people and that consequences are essential in teaching individuals the difference between right and wrong. At the beginning of the novel, Bryon willingly broke the law with Mark and did not feel bad about how his actions hurt others. As the novel progresses, Bryon matures and begins to feel empathy for those around him. He realizes that Mark has no conscience and does not understand the concept of right and wrong. Mark's willingness to harm others and not think twice about taking advantage of people concerns Byron. After hearing Mike's story, witnessing Charlie die, and finding M&M high off of LSD, Byron's perspective on life changes and he understands that Mark is a dangerous person. Mark's actions and disregard for others teaches Bryon that in order to make the world a better place, he must take responsibility and do everything that he can to stop those who are willing to hurt people. Although Bryon loses his best friend, he does the right thing by calling the police to lock up a criminal.