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Brooks Rattigan is a high school senior, with dreams of getting into Yale, while his father, Charlie wants him to go to the University of Connecticut. He works at a sub shop with his best friend, programmer Murph, but financing his college dreams proves troubling. Brooks seizes the opportunity to make some extra money by posing as the boyfriend of his classmate’s wealthy cousin Celia Lieberman of Greenwich. He finds he has a knack for dating with his adaptable personality. He meets beautiful but snobby Shelby and makes it his goal to win her over. Celia lies to Shelby and tells her Brooks is from Darien rather than working class Bridgeport.
He and Murph launch an app, selling himself as the plus-one for all occasions. After finding out that Celia has a crush on Franklin Volley (Blaine Kern II), they set up a plan to fake break-up, in which each of them gets to be with Shelby and Franklin. As business booms for Brooks, Murph feels neglected and cuts Brooks off. Celia sets up an interview for Brooks at Yale, and is upset when she finds out that he researched the Dean previously and lied to him in order to be liked. Brooks justifies it, saying that it's no different from what he's been doing with his app.
Celia realizes that Franklin is not the person for her, however doesn't tell Brooks. When they conduct their fake break up, she is hurt by his words, and slaps him. Brooks is under the impression that it's all an act.
Following the "break-up", Shelby asks Brooks to accompany her to her school formal. The two find it difficult to relate to one another and struggle to hold a conversation. At the formal, Brooks sees Leah, a girl who had used his app to "practice dating." She reveals all about Brooks' app to Shelby, who is offended and disgusted by the theme of Brooks' app. He reveals to Shelby that he's not from the wealthy town that she believes he's from, and that he needs the money to go to Yale. She calls him a liar and leaves.
Brooks approaches Celia, who's also at the dance. She declines his offer to dance, and says that she's not a backup. He returns home and talks to his dad about what's been going on in his life. His dad reminds him that nobody truly knows who they are, and admits to Brooks that he is proud of who his son is becoming.
Brooks decides to accept UConn's offer, because if he has to pretend to be somebody else to go to Yale, he doesn't want to go. Brooks makes up with Murph. He also meets up with Celia and writes a letter to her, where he reflects on how his previous ambitions were to drive the fanciest car, go to the nicest school, and date the prettiest girl. But he's revealed that these ambitions made him a bad friend, an ungrateful son, and a self-obsessed person. He writes that the times when he felt the most himself was when he was with Celia and wants to be with her.
Celia visits Brooks at home and apologizes for slapping him. The two then go to the sub sandwich restaurant which has been retrofitted for a party, with Murph and Tuna Melt in attendance. Murph reveals his admission to UConn and Brooks reconciles with Celia, sharing a kiss. The four then continue to dance until the end of the film.