The movie opens in the year 1951 in Enniscorthy, a medium-sized town in County Wexford in southeast Ireland. Eilis Lacy (Saoirse Ronan) (pronounced EYE-lish) is a young woman in her early 20s who seems unhappy as she prays in church with her mother Mary (Jane Brennan) and older sister Rose (Fiona Glascott). She then goes to work at a bakery store, run by the mean old Mrs. Kelly (Brid Brennan), who is very judgmental and critical. Eilis, who had tried to talk to Mrs. Kelly before her shift started but was rebuked, tells her that she is going to America. Her sister contacted a priest in America to sponsor her. She says she can work until she leaves, but is not permitted to do so. Mrs. Kelly guilts her for leaving her sister behind to take care of their mother for the rest of her life.
Eilis is at home at dinner with her mother and Rose, who does not think so highly of Mrs. Kelly either. They talk about the weather and about Eilis buying clothes.
That evening, Eilis meets up with her best friend Nancy (Eileen O'Higgins). They discuss boys, particularly one boy, George (Peter Campion), who seems to like Nancy. Eilis admits that she didn't put in more effort because she's going away soon. While the two girls are at a social, they awkwardly stand off to the side while couples dance. George comes over to Nancy and asks her to dance. Eilis watches then and then leaves.
As Eilis is packing, Rose is surprised that she owns so little and wishes she looked after her better. This is part of the reason why Rose is sending her, because she can't buy Eilis a future.
Eilis sets sail while her mother and sister watch, though their mother leaves early. Eilis and Rose cry while they blow each other a kiss. The ship appears to be full of mostly young people, being sent off by the older ones.
On the ship, Eilis finds her room, with a bathroom being shared by two girls next door. Her roommate, Georgina (Eva Birthistle), soon comes in to kick her off of the bottom bunk. She declares the cabin Hell and that she should not have come home from America, claiming that she has already traveled to the USA and back to Ireland and is now going back to stay. She invites Eilis out of the room for a cigarette, who refuses saying that she doesn't smoke. Georgina tells Eilis she'll be back soon, unless she finds a man from first class to smoke with.
Later, Eilis is the only one eating in the dining hall. She gets sick in the middle of the night and desperately has to use the bathroom, but the other girls had locked it from their side. She leaves her cabin and is forced to use a mop bucket to defecate in and when she comes back to the room, she throws up in a bucket there as well.
When Eilis wakes up, her roommate is taking care of her. Georgina forces her way into their bathroom and then blocks the others using it with her suitcase. She then goes to get Eilis water.
A day or two later, Eilis and Georgina talk outside on the ship's deck about her going to America. She is feeling better, as it's been hours since she's eaten. Her roommate tells her about America, what it's like, about letters, and how she'll meet people easily where she'll be living, in Brooklyn. When Eilis is eating again, her roommate is shocked by how pale and sick she still looks. She tries to dress her up so she won't be quarantined once she arrives.
At immigration, Eilis looks quite different and lovely. Georgina's voice-over tells her how to get through, which involves thinking like an American. As Eilis steps out of a blue door, she is bathed in light.
Eilis finds her new place, which is a boarding house where four other young Irish women live, which is run by the delightful gossipy, Mrs. Kehoe (Julie Walters).
Eilis walks to Bartocci's, the department store where she now works. As she is putting her stuff away in her cubby locker, she is approached by another girl who tries to strike up a conversation. It's not just other girls Eilis is awkward with, but customers as well. She is told by her supervisor Miss Fortini (Jessica Pare) that she has to treat every customer like a new friend. Eilis keeps to herself and seems awkward. It is clear she misses home.
On the day, Eilis had forgotten to check for mail, it turns out she had gotten letters from home. As soon as dinner is done she rushes up to read her letter, from Rose. She cries as she reads the letter.
At work, a crying Eilis is still being watched by Miss Fortini. The supervisor asks her if she's crying because it's her time of the month, which it's not. She tells Eilis to take a few minutes off and sends her to the staff room. While there Eilis is approached by the Irish priest who had sponsored her, Father Flood (Jim Broadbent). He had been told she was doing so well that he didn't think she would need looking after. He lets her know that she has enrolled her in a bookkeeping night class at Brooklyn College. He says that they need Irish girls in Brooklyn, and she wishes she were back in Ireland. He assures her that she will feel wretched at first, but that it will then move on.
The class does not seem to be an easy one, where they are learning about a Supreme Court case. Eilis is interested and doing well, though, and earnestly takes notes. During the break, a male classmate shares with her how hard he finds it.
During dinner at the boarding house, they discuss their Christmas plans. Two of the brash women, Patty (Emily Bett Rickards) and Sheila (Nora-Jane Noone), make fun of Eilis, since she is going to be serving homeless men at the church who have no other place to go.
At the church, Father Flood explains to Eilis when she asks that these men don't go home to Ireland because there is nothing there and they've lost touch. As a thank you, a man stands up to sing beautifully in Gaelic, which evokes emotion in Eilis as well as the other men. The music continues overhead during the film as the meal continues and is then cleaned up.
Eilis walks home where she is greeted by Mrs. Kehoe. As the two have a drink, Mrs. Kehoe explains that with one girl leaving, Eilis will be offered the best room in the house, the basement, since she is pretty and sensible. The other girls indeed look differently at Eilis during the next dinner. Patty and Sheila will be going to the movies instead of to a dance. Mrs. Kehoe remarks that hopefully they will have better luck with their new boyfriends than the ones before. Eilis says she will take the new girl, Dolores (Jenn Murray), to the dance.
Sure enough, the two girls just sit at an empty dance. Dolores talks about wanting to meet a guy. Patty and Sheila who had said they would go to the movies end up coming to the dance. They bring Eilis to the ladies room to put some lipstick on her and remarks that she looked like she had been milking cows.
Eilis is dancing with a boy who is teaching her, though he quickly moves on to another girl. She is then approached by a boy, Tony (Emory Cohen), who had been watching her and they dance, including to a slow song. Tony asks Eilis where she lives and asks to walk her home. She says yes and then explains why, so he won't get the wrong idea. She points to Dolores and says that he'd be rescuing her from Dolores.
While they're walking, he admits that he's not Irish, which she already knew. He explains that he's fully Italian-American. He came to the dance because he really likes Irish girls. Eilis makes him nervous as she teases him about if he danced with anyone else. Once Eilis arrives home, he asks to take her out on a date next week. She accepts and says goodnight. He watches her go in and leaves.
While Tony and Eilis are out to eat on their first date, Eilis tells him about school and how she wants to be a bookkeeper and eventually an account. She wants to know about his plumbing work, but he would rather hear about bookkeeping, which she goes quite in depth about. She also tells him about Rose. She's been talking so much though that she hasn't eaten much.
At the boarding house, Patty and Sheila are all knowingly smiling at Eilis. Mrs. Kehoe criticizes them and says Eilis is right not to say anything to those gossips. When they make fun of Tony's shoes, Mrs. Kehoe says she's going to ask the priest to talk about giddiness, the 8th deadly sin.
Another time later, Tony is waiting outside of class for Eilis to take her home. While they're riding on the trolley together, he dances around the question before asking her if she wants to meet his family. He tells her Italian food is the best in the world, which she's never tried. They also make plans to go to the movies.
At work, Eilis is now in a much better mood and easily talks to customers. Miss Fortini notices and says she's a different woman. Eilis offers it's because of Tony. Her supervisor says that since he doesn't talk about baseball or his mother too much she should keep him.
At the boarding house, Patty and Sheila teach Eilis how to eat spaghetti, which she is trying to do with a fork and a spoon.
Eilis writes to Rose about Tony, but asks that she not mention him to their mother. She tells Rose that he helps her feel like she has a life there that she didn't have before.
Tony warns Eilis about his younger brother Frankie, who seems to be looking forward to causing trouble, though the family had tried to talk him out of it. At dinner, Eilis is much more of an expert at eating spaghetti, and even tells Tony's mother that she took classes. Frankie then bursts out that they don't like Irish people since one of his brothers got beaten up by an Irish street gang. He is then taken aside by their father.
While Eilis shares how she is going to college for bookkeeping and working at the store, Frankie comes back to admit that he was a rude idiot, and they all applaud.
One of Tony's brothers asks Eilis if Tony has talked to her about baseball, and they say that's how she'll see him in the summer if she goes with him. Tony is a Dodgers fan and jokes how it will be bad if their kids are Yankees or Giants fans. Frankie points out that Eilis isn't laughing and so must be a Yankees fan.
Tony walks Eilis back home. Once they arrive, they hug goodnight. Tony says that he loves her. She thanks him for a lovely evening but is unable to say it back.
While Eilis is looking at herself in the bathroom, another girl accidentally comes in. Eilis asks her why she isn't married, which turns out to be because her husband met somebody else and left her. When Eilis asks if she'd get married again, she says she, of course, would. Eilis isn't sure if somebody has asked her.
The next time Eilis comes out of class, Tony isn't there. She is about to head home on her own when he shows up late. He seems awkward about last time and even though Eilis wants to talk about it, he changes the subject and talks about the job he had right before he came to that dance. She insists she talk though and lets him know that she loves him back. He was worried they were going to have a totally different kind of talk. Eilis asks him not to talk about their kids being Dodgers fans.
Rose writes to Eilis, thanking her for her nylons and talking about her golf tournament. She can't wait to see her one day. Over dinner with their mother she is crying.
Eilis brings her grades to Father Flood and says she has saved up enough money to pay for them on her own now, and to pay him back. He remarks how much she's changed.
Mrs. Kehoe points out how Eilis is the first girl to have passed exams while living at her house. Eilis shares that Tony is taking her to Coney Island to celebrate. The girls tell her that she'll need to think carefully about her costume. Even Mrs. Kehoe agrees, since it'll be the most he'll have seen of her, and she won't want to turn him off.
Eilis' supervisor helps her with bathing suits and comments on her needing to shave down there. She does compliment her full behind though and helps her into a nice green suit.
Tony and Eilis seem to be enjoying Coney Island together. At the beach, though, Tony has to hold up a towel so that Eilis can change into her bathing suit. They certainly enjoy swimming and kissing in the water.
Eilis writes to Rose to let her know she's really happy in America for the first time, and that it's because of Tony. He took her to a Dodgers game, and she's looking for office jobs. She'll even soon be able to afford coming home for a visit.
In Ireland, Eilis and Rose's mother knocks on Rose's door and upon opening it, finds her dead.
Back in New York, Eilis is approached by Father Flood while she is at work who tells her that her sister Rose had died back in Ireland. He explains that she likely knew she was sick with a terminal illness but didn't tell anyone. Eilis is upset that they're burying Rose without her being there, and she believes she'll never see her sister again. Father Flood says he thinks she will and that her sister will be watching over her.
When the phone rings, Eilis hesitates but picks it up. It is her mother who tells Eilis about the funeral. She makes Eilis cry though with how she has no one. Father Flood comforts her.
Eilis cries in Tony's arms while they're out to eat. He understand that she wants to go back to Ireland for her sister's funeral, but is afraid that he won't see her again. He says he wants to take her somewhere.
Tony takes her out to Long Island, to an open space where he hopes to build houses for his family and be in a building company with his brothers. He asks Eilis to at least think about it, but she says she doesn't need to.
Eilis can't sleep. Her mother back in Ireland is praying the rosary at church dressed in all black. Eilis gets up to go to Tony. She explains she needs to go home for just a month or two. He asks her to marry him before she goes. She's hesitant, but he says if they don't he'll go crazy.
Eilis brings Tony to her room where she warns him if he makes any noise she'll get evicted. They make love, and when done they hear footsteps, though they fortunately stop. Eilis has Tony stay the night.
The two are all dressed up to get married at City Hall. Tony plays ball with a little boy there and strikes up a conversation with his Irish father. He wants Eilis to meet them, but their name is called. When Tony drops Eilis off at home, they wonder what their children will think about such a story.
Some time later, Eilis is back home in Ireland. As she walks out of church with her mother, she sees Nancy, who is engaged to George. Her mother already knew but wanted Eilis to hear it from Nancy. Eilis is invited to the wedding, which is set to take place after Eilis plans to leave, but her mother says she can stay.
Eilis brings flowers to Rose's grave, which she is buried in the same plot as their late father (Note: the gravestone lists Eilis and Rose's father, Thomas Lacy, death on March 12, 1947, and Rose's death is listed as July 1, 1952). Eilis talks to Rose's gravestone about Tony and cries about how she wishes everything were different.
They go through the letters of condolences with Eilis wondering if she'll keep going back and forth sending thank you notes. Nancy is there, and her mother sends her off.
Nancy isn't there with their other friend Annette though, as Eilis had been expecting. Rather she and George have brought another young man along, Jimmy.
Once they arrive at the rugby club, Eilis asks why Jimmy is there, who had broken off an engagement with another girl because he didn't think she was serious enough. She reminds Nancy that she's going back to the United States, but Nancy assures her she can have fun before they go back.
While walking with Jimmy, he tells Eilis that he went to Rose's funeral and that her death was the saddest thing that happened in town. His mother knew her since they were in the golf club together. While talking to Nancy, George, and Jimmy, Eilis distinguishes how she lives in Brooklyn rather than Manhattan, which isn't very glamorous. Since she wants to be a bookkeeper like her sister, Jimmy says she should do some work before she goes back. George accuses him of wanting Eilis to stay, but Jimmy says he's just thinking of Eilis.
Her mother is still up and asks about her evening and asks about Jimmy. He'll have his house to his own soon, and her mother says he's a catch. Eilis just says goodnight.
Eilis returns home from shopping. She is told that she needs to go straightaway to her sister's old place of work to fix an accounting issue. It's a terrible mess for Eilis to work out. She makes it clear she'll make a system for the person after her and will not be staying.
The boss comes into compliment Eilis. He asks about her certificate and wants her to continue working there part-time and that they will speak again, even though she's going back. The grave is now filled, and Eilis continues to bring flowers.
Eilis goes to the beach with Nancy, George, and Jimmy. Since it's so different from Brooklyn, Jimmy wants to know if they must seem backward to Eilis, but she tells him they seem calm, civilized, and charming.
Jimmy tells Eilis there will be a prize in Rose's name at the local golf club. They want Eilis to present the first prize and Jimmy's mother also wants to meet her. Eilis mentions she wishes it were like this before Rose died.
While the other three have to change into their bathing suits under their towels, Eilis already has her bathing suit on. They remark it must be an American trick.
Tony wants help from Frankie writing out his letter to Eilis, since he's written so many, and she's only written back once. While Tony's letter is read aloud, Eilis seems to be getting along just fine in Ireland, including with Jimmy.
Eilis comes home to find a letter from Tony. She doesn't even read it but rather puts it in her drawer with the rest of the unopened letters.
Jimmy and Eilis are having lunch. She remarks that he looks different and jokingly ask if he's come to trick her. He admits that his life must seem very easy to her and others, but it doesn't feel like that. He tells her how he wants to go places and how he doesn't want to die having never left Ireland.
Eilis meets Jimmy's parents. Eilis makes an awkward comment about how Jimmy will not be alone forever. She tries to fix herself, but his mother just changes the subject.
Eilis reads all of the letters and begins to write one to Tony but cannot think of what to say.
Eilis and her mother go to Nancy's wedding with Jimmy. He offers to save them seats and Eilis and the mother remark what a gentleman he is and her mother mentions he came along at the right time.
Jimmy looks happy until he sees Eilis is emotional.
Eilis is told by an older woman that things are working out well for her. She tries to put off talk of Jimmy. But the woman presses on and hints that they'll soon be married.
While Jimmy and Eilis dance at the wedding, Eilis seems conflicted. Jimmy wants to talk to her about their future. He wants her to stay with him in Ireland, though he puts off proposing for another time since he doesn't want to bombard her. Eilis thanks him, is grateful and flattered, she just envisioned a different life. He understands but says her life could be just as good or better. They embrace.
Eilis is fetched by one of her former co-workers to go back to her old boss, Mrs. Kelly, to talk. She wants to know how she's been getting on with her job and Jimmy, but Eilis says it's just talk. She shares how a customer, Mrs. Brady, has a relative living in Brooklyn and so she knows Eilis was getting married.
Eilis says she has forgotten what this town is like and asks what Mrs. Kelly was asking her to do and assures her that her name is her married name.
Eilis composes herself outside and walks over to make the next reservation for New York.
Eilis' mother is just chattering away until she notices Eilis and thinks something happened with Jimmy. Eilis breaks down and apologizes for not telling her about Tony, but she wants to be with him. Her mother doesn't seem to mind and just wants her husband to be nice and wonders when she is going back, which is tomorrow. She then goes to bed and only wants to say goodbye now and only once. Her mother hopes she'll write about him and says goodnight.
Eilis looks around her room as she packs up her things. She looks out the window as she leaves but seems sure of herself.
Jimmy reads a note sent under his door. Eilis is on the boat with a similar situation as the beginning with a young girl asking her about America. This young girl is going to live in Brooklyn as well. Eilis gives her similar advice.
In the final scene, Eilis is there waiting for Tony coming out of work. Her voice-over discusses finding someone who'll make her realize that's where her life is. They're basked in light as she hugs Tony. Fade out.