In a school in small-town west of Ireland, Connell is a well-liked, good looking and athletic student. Marianne is a proud, intimidating and unpopular loner who actively avoids her classmates and questions her teachers' authority.

An attraction sparks between them when Connell comes to collect his mother Lorraine who works at Marianne's house. A deep connection grows between the two teenagers - one they cannot stop but that they are determined to conceal from their peers.

Connell and Marianne's relationship continues in secret and they sleep together for the first time. At school, Connell makes excuses for why he didn't see his friends at the weekend and evades questions on his mother's job at Marianne's house. A girl in his friendship group, Rachel, gets impatient with Connell's lack of interest and tensions build for Connell.

Meanwhile, Marianne must find a way to explain her unplanned absences to her prying brother Alan.

The night of the Debs is approaching and the girls at school have organised a fundraising event. Marianne has been asked to help and is out of her comfort zone.
At the club, one of Connell's friends Karen does her best to show Marianne some kindness but Rachel behaves coldly towards her.

The evening turns sour, when one of the boys' older friends gropes Marianne. She leaves the club, upset by the incident and Connell offers her a lift home. Marianne stays over at Connell's house and bumps into Lorraine as she leaves the next day.

Connell becomes anxious when his friends tease him about Marianne, suspecting they've
guessed about their relationship. Fuelled by anxiety, Connell makes an unexpected decision that hurts Marianne and causes a rift between him and his mother, Lorraine. Marianne withdraws from school, causing Connell intense guilt.

Months later, school has finished and Connell arrives at Trinity College, Dublin. He has found a room in a house-share and meets his new housemate Niall. Despite his efforts, he feels out of place and struggles to articulate himself when surrounded by his eloquent and wealthy peers.

Connell reluctantly accepts a party invite from new classmate Gareth. Feeling intimidated by the fashionable crowd in attendance, Gareth offers to introduce him to someone else he knows from Sligo and so he comes face-to-face with an unexpected guest.

Connell and Marianne start socializing as friends in Dublin. She introduces Connell to her group and one of her friends, Teresa is interested in him. Marianne is still dating Gareth. but her interest in the relationship is starting to fade. Marianne eventually finishes things with Gareth and makes a drunken pass at Connell.

Connell and Marianne are back together. Connell has gained confidence in college; giving presentations in class and getting good grades but is reluctant to show Marianne affection in front of her friends. Will the two be able to admit that they are together?

Things take a negative turn when Marianne goes home and argues with her brother Alan and Connell loses the part-time restaurant job that funds his life in Dublin.
Connell is torn when Marianne returns to Dublin distant and distracted, and finds himself once again, unable to share his worries.

Connell bumps into Marianne back in Sligo. It's the first time they have seen each other since they broke up. Connell asks Marianne how she is and tells her he has heard that she's now seeing Jamie. Despite the awkwardness between them, Connell offers to attend Marianne's father's anniversary mass the next day and they agree to stay friends.

Back in Dublin the two study hard for the Trinity Scholarship exams. On a night out after the scholarship results are announced Connell is mugged and turns to Marianne for help. On arriving at Marianne's house, he sees that he has interrupted a get together, which includes Jamie. The party breaks up, as the rest of group go out. Connell and Marianne are left alone. He reveals that he's seeing someone new and Marianne bursts into tears. They discuss the misunderstanding of their breakup, but it's too late now.

It's the summer holidays and Connell and Niall arrive at Marianne's family house in Italy. The obvious chemistry between Connell and Marianne causes friction with Jamie, despite Connell's evident happiness with his girlfriend Helen, who he clearly misses.

Peggy cooks the group a lavish meal but tensions run high. During dinner, Jamie drinks too much and picks a fight with Marianne. Connell breaks it up and attempts to soothe Marianne. Marianne stays in Connell's room that night to get away from Jamie. They talk and almost kiss, but Marianne puts a stop to it before it goes any further.

Marianne is on a student exchange program in Sweden for a year. Her breakup with Jamie has cost her some of her friends, but she is still in touch with Connell and Joanna. She is in a relationship with Lukas, a photographer who she meets at a college party.

Back in Dublin, Connell is enjoying his relationship with Helen and continuing his studies at Trinity. Word of Marianne's sex life with Lukas makes its way back to Dublin, and when Connell bumps into Peggy at a party she asks him about the gossip and whether things were like that with him and Marianne. Connell is upset by the gossip and worries about Marianne and whether she is ok on her own in Sweden.

Connell and Helen are out celebrating New Year's Eve in Dublin. Late that night, Connell receives a group text questioning the whereabouts of Rob. The next morning news comes through that Rob has died.

Connell struggles with the emotional fallout of Rob's passing. After the funeral he is distant with Helen only showing signs of responsiveness when reunited with Marianne. His mental health deteriorates when he returns to college, struggling to drag himself to lectures and he ends his relationship with Helen.

Eventually, at Niall's suggestion he begins counseling. Marianne is still in Sweden and attempts to reassure him over Skype. Through the counseling and Marianne's friendship Connell begins to see a way forward.

Marianne has returned to Carricklea for the summer. She and Connell spend their weekends together, even though he is still living in Dublin. After a night out together, they start to discuss their friendship and inevitably their attraction. When they end up in bed together, Marianne makes a request of Connell that upsets him. He refuses, and she leaves, ashamed.

At home, Marianne is provoked by Alan. She tries to get away from him, but he tries to force open the door and it hits Marianne in the face. She calls Connell, who immediately drives over to pick her up. He threatens Alan and promises Marianne that he'll never let anyone hurt her again.

Marianne and Connell are back in Dublin and together as a couple. Life is calm and they are at ease with each other. Then Connell receives news that he has been offered a place to study in New York. Neither of them know how to process the news and so they decide to let it sit. Life goes on.

Marianne stays with Connell and Lorraine for the Christmas holidays, and is embraced by their extended family. During New Year's celebrations, Marianne and Connell kiss in front of their old schoolmates.

Back in Dublin, Marianne's mother has asked her to take out the last of her things from the flat that she lived in at the start of college. Alone in the house together, they discuss the offer from the New York university once more.