The following version of the book was used to create the guide: Creedle, Laura. The Love Letters of Abelard and Lily. Harcourt Publishing Company, 2017.

Lily Michaels-Ryan, the narrator and main character in the novel, is a teenage girl who grows up in Austin, Texas with ADHD and dyslexia. Her neurological differences cause her to be quite impulsive, resulting in many forgotten assignments and skipped class. One afternoon in class, a handle on a sliding door in her classroom begins to wiggle and screech, and Lily feels the need to yank on the handle to stop the noise. Unfortunately, the handle breaks, and her antics land her in the vice principal's office with Abelard Mitchell, another neurodivergent student at the school who deals with Asperberger's. Lily has always been fond of Abelard, and in another impulsive decision she kisses him, leaving him dazed and confused.

Lily and Abelard end up in detention together the following day, where Lily picks up a portrait of Abelard drawn by her classmate, Richard. Lily's sister, Iris, posts the portrait online for a school project and the photo gathers some attention online along with some hurtful comments. After apologizing to him for subjecting him to online bullying, the two are drawn together by their shared love for medieval literature and begin to exchange quotes every night from The Letters of Abelard and Heloise, a collection of love letters between unrequited lovers.

Their relationship continues to grow, and they start to spend time together in person. They long for time alone with each other, but Abelard's mother constantly checks in on them. Lily learns that Abelard's condition makes physical contact very difficult for him and she comes to realize that they will not be the usual high school couple.

Despite things going well for her personal life, Lily continues to struggle with school. After failing her geography assignment because of a paperwork formality, she decides her best option is to fail out of school and move to Portland with her dad. However, her lack of interest in the things she usually cares about causes people around her to be worried, and the school puts her on suicide watch. She tells Abelard she is broken, but he tells her that she is perfect and complex, comparing her to a fractal. The two confess their love for each other, and they continue to spend more time together.

When Lily's report card arrives in the mail, she uses her failing grades as an excuse to move to Portland with her dad to be homeschooled. Her mom does not bite on the idea, and instead suggests that Lily talk to a surgeon about an electrode treatment for ADHD. Lily refuses to go see the surgeon unless her mother promises to let her and Abelard visit her dad in Portland. As a compromise, her mother allows her one phone call with her dad, but unfortunately Lily learns that he has a new family. Upset that her plan was no longer viable, she agrees to meet with the doctor.

That weekend, Lily arrives late at Abelard's house to watch a movie. Already anxious because of Lily's lateness, Abelard becomes overwhelmed by his dad talking over the movie and starts pounding his head on the table. Lily flees to the kitchen in an effort to give Abelard some relief but breaks a glass pitcher. She tries to hide the evidence, but cannot find the garbage, so she runs to the backyard in an attempt to escape. However, her efforts are unsuccessful, and she runs into Mrs. Mitchell, who calms her down. Later, Abelard confesses that he was distraught over thoughts that she was not coming, but Lily reassures him she will never be late again.

Lily and her mother go to see the doctor. After the Lily passes an informal test, she and the doctor discuss the surgery in private. The doctor explains that the surgery will help her focus, and she might even be able to go to college. Lily agrees that the surgery is the best option, and for the first time she becomes excited about the future.

After things start to change for the better for Lily, Abelard explains that he has been accepted at the Isaac Institute in New Mexico. Abelard tells Lily he will not accept the offer unless she comes with him, but Lily does not want to be the reason he turns down college. Instead, Lily lies to Abelard and tells him she is moving to Portland to be with her father. The situation between them becomes touch-and-go, and when Abelard tells Lily that he has met a nice girl on his floor of residence who reminds him of her, she becomes too upset to think rationally, she breaks up with Abelard.

The following day, Lily's dad pays a sudden visit to see her before the surgery. During their conversation, she comes to realize that he is not everything she expected him to be; however, the two are more alike than she thinks. The visit is closure, and reassurance for her that surgery is the correct option.

After holding out for a while, Lily breaks down and texts Abelard to tell him the truth about everything; how she lied about moving to Portland, and that she is getting brain surgery. Although he is angry, he takes the bus back to Austin so they can talk. During his visit, the two plan to take the train back to New Mexico and continue their lives. However, as Lily gets a drive from her mother to the train station, she realizes that leaving is not what will make her happy in the long run, and decides to stay home and get the surgery. Although Abelard is crushed by Lily's decision, he understands it is what is best for her. He suggests a long-distance relationship, and realizing they need each other, Lily agrees - while they cannot be together in person, their love story can continue.