In 2017, 23-year-old Lucy Albright prepares to meet Pippa and Jackie at their college friend Bree’s wedding weekend. Lucy is very anxious about seeing Stephen DeMarco, her ex-boyfriend. The scene shifts to 2010, when Lucy, newly arrived at Baird College outside L.A., first meets Jackie, Pippa, and Bree. They attend parties where Lucy encounters Stephen, a junior who is immediately attracted to her. Over the course of the fall semester, Stephen pursues Lucy, and in time, Lucy grows to trust Stephen. Stephen, incapable of forming emotional bonds, sees people as commodities. He pursues Lucy even though he’s in an on-again, off-again relationship with a fellow junior, Diana Bunn. After a party one night, Lucy shares her deepest secret with Stephen: the Unforgivable Thing that her mother, CJ, did when Lucy was in high school. Believing that their shared disclosures of their problems with their mothers indicates an emotional connection, Lucy feels safe submitting fully to Stephen, and they sleep together for the first time. 

Over the ensuing months, Lucy and Stephen sleep together and sometimes date, though Stephen remains connected to Diana. Lucy is sexually satisfied with Stephen, but their emotional relationship is troubled. Pippa accuses Stephen of manipulating Lucy. Meanwhile, Lucy becomes increasingly anorexic, severely restricting the calories she consumes and exercising to the extreme. Lucy and Stephen are both from Long Island, and as spring semester ends, they make plans to get together during the summer. However, in May, at the Hawaiian Luau party, Stephen abandons Lucy to be with Diana. Lucy spends the summer heartbroken, getting thinner and thinner. Her anger at CJ keeps her emotionally distant from her family, and CJ is very worried about her.  

Stephen and Diana are together in the first half of Lucy’s sophomore year. Stephen applies to law schools but worries that his DUI arrest before college will keep him out of top schools. Unable to move on from Stephen, Lucy retreats from the social scene. While alone in her dorm in January, as she is thinking about Macy Peterson, a girl she knew in high school who died in a car accident, Stephen shows up at her door unexpectedly. They sleep together that night, and their sex remains fantastic, although Lucy hates sneaking around to avoid Diana. As he feared, Stephen is rejected from all the law schools he applies to, and he takes a job at a brokerage firm in New York City. He tells Lucy he will break up with Diana at graduation, and they then can be together. Lucy discovers that she has been accepted into a prestigious study abroad program, Writers on the Riviera, but she turns the program down so she can attend Stephen’s graduation party. Lucy enjoys a summer of seeing Stephen in New York. At the end of the summer, Diana, who had planned to visit before her move to Saint Louis, breaks up with Stephen for good. Stephen is unwilling to commit to a long-distance relationship with Lucy but says he will see her at Thanksgiving break. 

After Lucy returns to Baird, Stephen begins dating Alice, and Lucy falls into a depression for much of her junior year. She develops feelings for a classmate named Billy, but she is too obsessed with Stephen to date anyone else. Meanwhile, Stephen is becoming bored with Alice. He has also begun having panic-induced flashbacks of the smell of Bubblicious gum, red hair, blood, and the Cranberries song “Zombie.” Jackie discovers that Lucy lied about not getting accepted to Writers on the Riviera, and she is angered by the way Lucy has allowed Stephen to rob her of her zest for life. Lucy’s depression is bad enough that when she gets home to Long Island for the summer, CJ forces her to see a psychiatrist, Dr. Wattenbarger. On the day of her first appointment with him, Lucy steals the box of jewelry CJ inherited from her beloved Aunt Marilyn and disposes of it, selling the valuable items and throwing the rest into the East River before donating the money anonymously to St. Thomas Church.  

Lucy’s senior year is happier, thanks to Dr. Wattenbarger. She is dating Billy when Stephen, now tired of Alice, visits to show the campus to his sister Sadie. Lucy and Stephen have sex after a concert, and Lucy decides to move to New York rather than live in L.A. with Jackie and Pippa. After graduation, she takes the first job she finds so she can get an apartment in the city with Bree and be with Stephen. Stephen, now in law school, first tries to live with Alice while sleeping with Lucy on the side. Lucy refuses to participate in being involved with Stephen while he still resides with Alice, so Stephen ends his relationship with Alice. By the fall, his growing irritation with Lucy mirrors the boredom and irritation he felt with previous girlfriends. The narrative shifts to Stephen’s recollection of the death of his secret high school paramour Macy Peterson. Though Lucy, Macy’s friend and high school classmate, and others believe that Macy was alone in the car when it veered off the road, Stephen was actually the driver. He was drunk and receiving oral sex from Macy when he lost control of the car and crashed. Realizing his involvement would mean the end of his plans for college and beyond, Stephen swapped Macy’s lifeless body to the driver’s seat and walked home.  

Bored with Lucy, Stephen goes on a date with Jillian and is seen by Lydia’s friend Charlotte. After Lucy confronts him about it, he avoids her and then breaks up with her. Bree is so worried about Lucy that she calls CJ, who comes to take her to the Cape. Once there, Lucy finally confronts her mother over the Unforgiveable Thing: her affair with Gabe Peterson, Macy’s brother. CJ apologizes, and the two begin to heal their relationship. Back in 2017, at Bree’s wedding, Lucy is initially upset to see Stephen with Jillian, now his fiancée, but ultimately realizes she is no longer interested in him, feeling only apathy in his presence. She calls CJ and tells her she wants to move away from New York and start fresh. CJ encourages Lucy to dream big, saying they can talk about it more when she comes home for a visit after the wedding.