During the 2011 ICCA Finals at Lincoln Center, Barden University’s all-female acappella group, the Barden Bellas, lose to their all-male rival group, the Barden University Treblemakers, due to junior member Aubrey Posen vomiting on stage in the middle of her solo. Four months later, newly arrived Barden University freshman Beca Mitchell has no desire to attend college but is forced to do so by her father, a literature professor at the university, with whom Beca has a strained relationship. Wishing she could instead pursue a career as a music producer in Los Angeles, Beca spends her time making mash-up mixes of songs and takes up an internship at the school radio station, where she gets to know fellow freshman Jesse Swanson.
At the university’s activities fair, Beca is invited to join the Bellas by seniors and current co-leaders Aubrey and Chloe Beale, but she declines. Later, Chloe discovers Beca’s talent for singing and convinces her to audition. Beca is late to the audition, but Chloe allows her to perform for them anyway. She auditions with a rendition of "Cups (When I'm Gone)", getting her into the Bellas along with Cynthia Rose Adams, Stacie Conrad, Lilly Onakuramara, Jessica Smith, Ashley Jones, Patricia "Fat Amy" Hobart, and Denise. Meanwhile, Jesse joins the Treblemakers.
The Bellas participate in the 2012 ICCA Regionals, where, at Aubrey's insistence, they perform the same medley that helped the Bellas advance to the Finals the previous year. In spite of their stale setlist, the group manages to place second, sending them to the Semi-Finals. After the competition, the Bellas try to break up a fight between the Treblemakers and the Tonehangers, a male alumni a-cappella group. Beca and Fat Amy accidentally smash a window with the Treblemakers' trophy, which alerts a police officer who then arrests Beca. Jesse contacts Beca's dad to bail her out, causing a rift in her relationship with both men.
Aubrey insists on performing the same medley a third time, despite Beca urging the group to be more daring. In the midst of their performance, Beca, hoping to reinvigorate the disinterested crowd, inserts an impromptu layering of "Bulletproof" into the set. Aubrey angrily berates Beca for this and accuses her of hooking up with Jesse, a rule breach punishable by ejection from the Bellas. Jesse overhears and attempts to deny it, leading Beca to snap at them both and quit the Bellas.
Despite the judges and crowd being impressed with Beca's improvisation, the Bellas do not advance to the Finals due to their third-place ranking behind the Treblemakers and the Footnotes. However, Jesse’s roommate Benji Applebaum finds out that Footnotes leader Timothy is a high school student and reports it, which disqualifies the Footnotes and allows the Bellas to advance to the Finals.
After spring break, Beca tries to apologize to Jesse, but he rejects her, accusing her of pushing away everyone that cares about her. During the Bellas' rehearsal, Chloe stands up against Aubrey's stubbornness, sparking a fight over the group's pitch pipe. However, Beca returns, apologizes to the Bellas for changing the set without Aubrey's permission during the Semi-Finals, and asks to be given a second chance. After all of the Bellas have a heart-to-heart conversation, Beca rejoins the group, and Aubrey relinquishes her half of the leadership to Beca.
Meanwhile, Treblemakers leader Bumper Allen leaves the group after being offered a job as a back-up singer for John Mayer. With Bumper gone, Jesse persuades the Trebles to let Benji join the group in Bumper's place, a position Benji was denied earlier in the film in spite of his impressive audition.
At the Finals, the Bellas perform a modern piece arranged by Beca, which includes "Don't You (Forget About Me)", a song featured in The Breakfast Club, one of Jesse's favorite movies. This acts as a more effective apology, and after the performance, she and Jesse kiss. The Bellas defeat the Treblemakers and win the national championship. Six months later, auditions for new members take place.
The Barden Bellas
Anna Kendrick as Beca Mitchell, an introverted and rebellious freshman who joins the Barden Bellas to appease her professor father.
Rebel Wilson as Patricia "Fat Amy" Hobart, a comical and confident singer from Tasmania.
Anna Camp as Aubrey Posen, the uptight and traditionalist co-leader of the Bellas.
Brittany Snow as Chloe Beale, the kinder and more civil co-leader of the Bellas.
Alexis Knapp as Stacie Conrad, a highly sexually driven member of the Bellas.
Ester Dean as Cynthia Rose Adams, a soul singer.
Hana Mae Lee as Lilly Onakuramara, a strange quietly-spoken student but a talented beatboxer.
Kelley Alice Jakle as Jessica, bubbly yet absent-minded.
Wanetah Walmsley as Denise, the group's original bassist.
Shelley Regner as Ashley, a beatboxer.
The Treblemakers
Skylar Astin as Jesse Swanson, an outgoing Barden freshman who hopes to one day become a film score composer.
Adam DeVine as Bumper Allen, the egotistical leader of the Treblemakers.
Ben Platt as Benji Applebaum, Jesse's roommate, friend, and an illusionist.
Utkarsh Ambudkar as Donald, Bumper's right-hand man, a lead vocalist, beatboxer, and rapper.
Michael Viruet as Unicycle
David Del Rio as Kolio
Others
John Michael Higgins as John Smith, a commentator for the ICCAs.
Elizabeth Banks as Gail Abernathy-McKadden, a commentator for the ICCAs.
John Benjamin Hickey as Dr. Mitchell, Beca's father, a professor at the college.
Freddie Stroma as Luke, the radio station manager who plays Beca's DJ mixes on the air.
Jinhee Joung as Kimmy Jin, Beca's Korean American roommate.
In addition, Joe Lo Truglio, Har Mar Superstar, Jason Jones, and Donald Faison appear as the Tonehangers, an older, long-graduated, a cappella group that gets into a fight with the Bellas and Treblemakers, while Christopher Mintz-Plasse plays Barden student Tommy, who organizes the school’s auditions.