The Poet X by Elizabeth Acevedo is written in poetic form from the viewpoint of Xiomara. Xiomara is a sophomore whose parents came to America from the Dominican Republic. They live in Harlem. Her mother thought she was going to be a nun until her parents sent her to America with Xiomara's father. X's father had been with many women and thought he was sterile until Xiomara's mother became pregnant with twins. When his children were born in January, Papi settled down to raise his son, Xavier, and his daughter, Xiomara. Xiomara's mom is still very religious. She makes Xiomara take a confirmation class, which she should have taken earlier, but her best friend Caridad was out of the country, so they waited. Xiomara is questioning religion at this point in her life even though Caridad seems quite devoted. Xiomara's mother is so religious that when Xiomara got her period when she was eleven, she tried using a tampon, and her mother scolded her, wondering if she was no longer a virgin. Xiomara expresses some of these feelings in her writings. A tomboy, Xiomara often has to protect her brother, whom she calls Twin, even though he is an hour older and much smarter than Xiomara as he attends a private school.

The book starts just as school is starting. One of Xiomara's teachers, Ms. Galiano, tells the class about a Poetry Club that sounds interesting to Xiomara. She knows she can't attend though because it meets on Tuesdays when she has to go to confirmation class. Her mother works as a cleaning woman. She has strong feelings about Xiomara not dating. Her first assignment for Ms. Galiano is to write about the most impactful day of her life. She ends up turning in a description of her twelfth birthday when Twin bought her a fancy notebook to use as a journal.