Marianne Sheridan and Connell Waldron attend secondary school in a small town in Ireland and will soon be going off to college. They are smart, like to read, are interested in world events, and care about social justice. Marianne lives with her brother and her widowed mother. They are a wealthy family, but her brother, Alan, is verbally and physically abusive toward Marianne, and her mother, instead of intervening, takes Alan’s side. Marianne has low self-esteem from years of internalizing her brother’s verbal abuse. 

Connell’s mother, Lorraine, works as a cleaner. She dropped out of school when she became pregnant with Connell and never told anyone who the father was. Lorraine cleans the Sheridans’ house twice weekly. Since she doesn’t drive, Lorraine needs Connell to pick her up when she is done, so Connell and Marianne see each other often. Connell is self-conscious about his origins, naturally shy, inarticulate in conversations, and indecisive.

At school, Marianne is deliberately antisocial and is considered a misfit, whereas Connell is handsome and athletic, and is therefore popular despite his private insecurities. Publicly, Marianne ignores Connell to avoid harming his social standing, but privately she tells Connell bluntly that she has feelings for him. Cornell has feelings for Marianne, too, but he thinks of their relationship as a private matter and not to be shared with the rest of the world. His feelings are partly based partly on wanting to protect his social status, but also he connects with Marianne in a way he does not connect with anyone else and does not fully understand. 

Lorraine is aware that Marianne and Connell are growing close and approves since she likes Marianne a great deal. After Marianne and Connell begin a sexual relationship, over time Connell becomes more willing to be seen with Marianne in public. The relationship ends abruptly, however, when Connell asks one of the school’s most popular girls, not Marianne, to the dance for graduating seniors. Lorraine is furious at Connell, and Marianne is so humiliated that she stops attending school. Connell is ashamed and wants to apologize to Marianne, but she ignores his calls and texts.

Connell and Marianne meet again while attending university in Dublin, when her current boyfriend, a political right-winger named Gareth, invites Connell to a party. Marianne and Connell reconnect and resume their relationship. However, whereas she is comfortable in the upper-class university environment and has become much more sociable than she was in school, Connell continually feels out of place at university. Connell’s sense of social inferiority leads to a second breakup with Marianne after a few months. Connell goes on to have a relationship with another student, Helen, who is more conventional than Marianne and condescends toward him. When Connell and Marianne both win a coveted scholarship, both his social standing and his financial situation improve. However, he still feels restless and unhappy at university.

Marianne has other relationships as well. She was a virgin before being with Connell, but during a relationship with Jamie, who is from a family even wealthier than her own, Marianne discovers that she wants to be dominated and physically abused during sex. When Marianne shares this fact with Connell, she assures him that her behavior with her boyfriend is an act. For Connell, she says, she really would do anything. Marianne continues to suffer abuse at the hands of her brother and mother when she is home for visits. She breaks up with Jamie after he behaves badly in front of others during a European trip. Connell comforts her, but she does not get back together with Connell. During a study year in Sweden, Marianne has another sado-masochistic relationship, this time with a photographer named Lukas. However, when Lukas mentions love, Marianne ends their relationship abruptly.

Throughout their relationship, violence repeatedly impacts Marianne and Connell in unexpected ways. Marianne’s relationship with Jamie suffers strain and Marianne and Connell are drawn closer together when Connell is mugged and Marianne tenderly sees to his injuries in front of Jamie and other friends. The suicide of a friend from school days not only sends Connell into a severe depression but also leads to Helen’s breaking up with Connell when she sees at the funeral how much Connell still cares for Marianne. During a later summer back home, Connell rushes to be with Marianne after Alan breaks her nose by smashing a door in her face. Connell threatens to kill Alan if he mistreats Marianne again.

By the time graduation from university approaches, Marianne understands that she and Connell have been good for each other, despite all the troubled times. Connell has developed a desire to write fiction professionally and is showing a romantic interest in Sadie, a past editor at the campus literary magazine. When Connell is accepted to a graduate program in creative writing in New York, Marianne tells him he should go, and that she will always be there for him.