Friendless student Ellie Chu lives in the remote town of Squahamish, where she makes extra money writing homework papers for her fellow students. She lives with her widowed father and performs most of his duties as station master and signalman. She is approached by Paul Munsky, an inarticulate football player, to write a love letter to Aster Flores, a girl at their school. Ellie is secretly in love with Aster and initially refuses, but accepts the work after learning that her home’s electricity will be shut off if they fail to make a minimum payment. Aster is dating Trig, a boy from a wealthy family, and Ellie begins a heartfelt correspondence with Aster in letters and text messages. The two connect over their shared interests in literature and art, though Aster believes she is communicating with Paul. Ellie’s English teacher encourages her to apply to Grinnell College, but Ellie plans to stay in Squahamish, despite being unhappy there, because she doesn’t want to leave her father.
Ellie sets up Paul on a date with Aster, but it goes badly because Paul is unable to converse with her like Ellie can. Ellie regards this as the end of their attempts, but after Paul defends her from bullies, she agrees to teach him about art and literature. Ellie and Paul begin to bond as they open up about their family struggles. Paul’s second date with Aster goes as badly as the first, but Ellie saves the evening by texting Aster while pretending to be Paul. Paul abruptly professes his romantic interest in Aster and later tells Ellie that he kissed her. Paul brings Ellie to a party and takes her to his home after she becomes intoxicated, where he sees letters in her bag addressed to food critics, presumably about his sausage creations.
Aster invites Ellie to spend the day together. She takes her to a hidden hot spring where they swim and share an intimate conversation. Aster reveals that Trig intends to marry her, while Ellie reveals that she does not believe in God and talks about her deceased mother. Meanwhile, at Ellie’s house, Paul and her father bond as they make sausages. Later, Ellie watches from her window as Paul and Aster kiss, and she decides to apply to Grinnell. After a game, mistakenly believing that she loves him, and having developed feelings for her, Paul attempts to kiss Ellie, but she rejects him. Aster sees them and walks away angrily. Paul realizes that Ellie loves Aster and, due to his religious upbringing, tells Ellie that it is a sin. When Paul delivers sausage to Ellie’s father, Ellie’s father and Paul have a conversation about love and asks Paul if he’s ever loved someone so much that he didn’t want to change anything about them. Trig proposes to Aster at a church service, but Ellie and Paul publicly interrupt the moment by sharing the true meaning of “love” using Ellie’s wording from a previous letter. Aster realizes that Ellie wrote the letters and text messages pretending to be Paul; she slaps Paul and storms out.
Before leaving for Grinnell, Ellie apologizes to Aster for deceiving her. Aster admits she may have had feelings for Ellie and that she is applying to art school. Ellie kisses Aster and tells her that she’ll see her in a couple of years. Paul sees Ellie off at the train platform and tells her that his sausages have received good reviews, and that he’ll keep visiting her father. As in Ek Villain, the movie they watched together, Paul runs besides the train as it leaves. Ellie laughs at his silliness, then observes the passengers around her as she goes off to start a new journey in her life.