Beca Mitchell (Anna Kendrick), a newly arrived freshman at Barden University, has no desire to attend college, but she is forced to do so by her father (John Benjamin Hickey), a professor at the university. Wishing to instead move to Los Angeles to pursue a career in music making, Beca takes up an internship at the school radio station, where she meets fellow freshman Jesse Swanson (Skylar Astin), who develops a not-so-secret crush on her. When Beca's father discovers she has not been attending classes, he comes to her with a proposition: if she joins a club and still does not want to remain at college after a year, he will allow and pay for Beca to move to Los Angeles. After some persuasion from group leader Chloe Beale (Brittany Snow), she reluctantly joins the all-girl a cappella group Barden Bellas. Aubrey Posen (Anna Camp), another group leader did not initially approve of recruiting Beca, but yielded nonetheless due to a desperate need for members and Beca's own talent. Joining alongside a group of other women who do not fit the Bellas' typical mold: the misunderstood Cynthia-Rose (Ester Dean), the sexually obsessed Stacie Conrad (Alexis Knapp), the awkwardly quiet Lilly Onakuramara (Hana Mae Lee), who speaks inaudibly, and the irresponsible and eccentric Fat Amy (Rebel Wilson).
At group rehearsal, Aubrey's uptight and controlling nature gets on the nerves of the other girls, especially Beca. She is constantly critical and insists that they use the same set list the Bellas have been using for years, which clashes with Beca's preference for originality and doing things different. Meanwhile, Beca hangs out with Jesse, who has joined the Treblemakers, the Bellas' main rival. Aubrey warns the group through initiation not to have any sexual contact with the Treblemakers, or they'll be forced to leave the Bellas. Beca plays several of her own mixes of contemporary songs for him, and he compliments her for her talent. After learning that Beca doesn't like movies, never even making it through a movie from beginning to end because she loses interest very quickly, Jesse tries to get Beca to watch the ending of The Breakfast Club, one of his favorite movies.
Meanwhile, the Bellas attempt to put aside their issues and participate in the Southeastern regional competition at Carolina University. In spite of their distasteful and old set list, the group manages to place second at the regional a cappella competition, which gives them the bid to semi-finals. Later, some incidents in brief scuffle after the competition leads to Beca accidentally breaking the windows of the hall's entrance, which in turn got her arrested by the police. Jesse tries to help by bailing her out by calling Beca's father, which infuriates Beca and turns their budding friendship sour.
On their way to the semi-finals, the group stops for gas where Fat Amy is hit with a burrito by Bumper Allen (Adam DeVine), the leader of the Treblemakers, as they drive past in their own bus. After the other girls clean her up, they continue on their way but run out of gas, as Fat Amy had forgotten to actually pump gas into the bus after getting hit with the burrito. They are then forced to call the Treblemakers to drive them to the venue, which then leads to some smack talk during the car ride there.
Once at the semi-finals, Beca notices how bored the audience is with Aubrey's traditional and overused arrangement, so she provides impromptu back-up, La Roux - "Bulletproof." The Bellas come in third in the competition behind the Treblemakers and the Footnotes; even though the audience seemed to enjoy Beca's remix, Aubrey becomes furious and yells at Beca, who quits the group after yelling at both Aubrey and Jesse, who just happens to be around trying to cool things down. When the Footnotes are disqualified after the discovery that their lead singer was still in high school, the Barden Bellas are called to Nationals in their place. The Bellas get back together after returning from spring break, with the notable exception of Beca.
During this time, Beca begins working as a DJ at the Barden radio station, playing her music on the overnight shift while also steadily distancing herself from Jesse. Upon finding a copy of The Breakfast Club and watching it in its entirety, however, she realizes how much she needs the Bellas and Jesse in her life. Her father convinces her to rejoin the Bellas, whom she finds falling apart during rehearsal, which was sparked by Chloe finally got tired of Aubrey's controlling nature and stood up to her. After having a heart-to-heart in which they revealed secrets about themselves, the Bellas appoint Beca as their group leader and decide to adopt Beca's more modern and original music style. Meanwhile, Bumper leaves the Treblemakers after being offered a job as a back-up singer for John Mayer. With Nationals approaching and the group short a member, Jesse becomes the new group leader and persuades the Trebles to allow Benji (Ben Platt), Jesse's roommate who auditioned together with Beca and Jesse but did not make it due to being nerdy and weird, to join the group in Bumper's place.
At the a cappella national competition at Lincoln Center, the Bellas sing a piece arranged by Beca. Once their performance is over, Beca reunites with Jesse with a kiss. Fast forward to six months later, auditions are beginning for the new year and the Treblemakers and the Bellas are preparing to select new members for their groups; Jesse and Benji are shown sitting at a desk in the auditions hall, followed by Beca and the Barden Bellas, along with their trophy from Nationals.
