The following version of the book was used to create this study guide: Jackson, Tiffany D., Allegedly. HarperCollins, New York, NY, 2017. Kindle AZW file.

When Mary Addison was nine years old, her mother was babysitting Alyssa, the three-month-old daughter of a friend and neighbor. By the time Mr. and Mrs. Richardson returned that night, the police were at the Addison home and Alyssa was dead. Alyssa had been brutally beaten. The investigation into finding the murderer led police to arrest Mary, though at nine years old, officials hardly knew what to do with her. Without proper representation and with her mother making decisions for her, she was found guilty of manslaughter. She spent the following years in jail, mainly in isolation because the crime was horrendous. However, she was too young to be safe among the other criminals, who were all much older.

The novel opens when Mary is 16 years of age. She is now living in a group home and she has hopes for the future, though she is facing some problems. She is in a trade school program but she is studying for the SAT in the hope of entering college. Because of her criminal record, she is going to have to have a near-perfect score. She manages to get a study book and is invited to participate in a study group toward that goal.

Mary has a boyfriend named Ted, and she believes they are perfect for each other. Ted also has a criminal record, but he is planning for their future. When Mary discovers that Ted is not devoted solely to her, she becomes determined to break off the relationship even though she is pregnant with his child. That pregnancy is another problem because Mary is still underage and is officially a ward of the state. She has few rights and knows that officials will likely take the child from her as soon as she gives birth.

Mary wants her life to change and on the advice of another girl in her group home, she contacts a lawyer who has always had doubts about Mary's guilt. The lawyer agrees to take the case and Mary finally tells the whole story. This version of events is very different from the version that Dawn told when Alyssa died. In this version, Dawn was furious that Alyssa would not stop crying and she shoved some of Mary's pills down the baby's throat to try to shut her up, beat the child, then tried to perform some sort of religious ceremony to bring her back to life. Dawn would not allow Mary to call for help and convinced Mary to take the blame, saying that Mary would barely get any punishment. Several officials immediately accept Mary's version of events. When Mary is attacked in the group home, officials also crack down on that situation and move Mary to a halfway house for pregnant girls and the possibility exists that Mary will be allowed to keep her baby.

As she leaves the group home behind, Mary thinks over the events as they really happened the night Alyssa died. She had tried to make the baby shut up and killed her in the process. She felt that Alyssa's mother would not be angry at Mary because, after all, Alyssa was being bad. Mary thinks about the baby she is carrying and knows that he will always be good, though she thinks it may not be a bad thing for someone else to care for him while she is going to college and getting herself in a better place. She also knows that most of the officials involved in her case now think she is innocent. After all, that is what happens when one is a really good liar.