The movie opens with the Barden Bellas about to perform at the ICCA championships. One of them, Chloe (Brittany Snow) is running late to the start of the performance. When she arrives she is reprimanded by the older Bellas. As they are about to perform, Aubrey (Anna Camp) tries to assure them she'll do a good job, but is shot down. The group sings 'The Sign'.. and all is going well, until the very end when Aubrey has a solo, and projectile vomits all over the stage and front rows of the audience.
Cut to the start of a new year at Barden University, and freshman Beca (Anna Kendrick) is arriving by taxi. While she is receiving directions and her Rape Whistle, a car passes her by, and another student, Jesse (Skylar Austin) is singing along to the radio, and catches her attention.
Beca finds her new dorm room, and meets her roommate Kimmy-Jin, who is hostile and unfriendly. Beca's dad, a professor at Barden then arrives, and asks her how she got there. It is revealed that Beca's parents have split up as she refers to her dad's new wife as her 'step monster', and seems very reluctant to be attending College. Beca's dad tries to encourage her to enjoy it, but Beca escapes to the Activities fair.
In another dorm room, Jesse is meeting his roommate Benji (Ben Platt), who is into Star Wars and magic.
At the Activities fair, we are introduced to the Trebles - one of the Campus singing groups, all male, who are talented and have a high opinion of themselves, especially Bumper (Adam Devine) the lead singer. Jesse and Benji approach the Trebles, as Benji is desperate to join, but Bumper is not impressed with his enthusiasm.
Across the campus, the Bellas are now down to just 2 members - Aubrey and Chloe, who are trying to recruit new ones. Due to their disastrous performance at the last Championships, they are not having much luck, and are even ridiculed by 'Baloney Barb' who has tried out for them previously. They are then approached by Fat Amy (Rebel Wilson), an outgoing and quirky student from Tasmania who doesn't hesitate to demonstrate her singing and dancing abilities. They then meet Beca, who quickly rebuffs their offer to audition, saying it is lame, which immediately puts Aubrey offside.
Instead of being interested in joining any of the singing groups, Beca has much more of an interest in remixing music on her laptop. She gets a job at the campus radio station, and much to her dismay, so does Jesse.
Her dad pays her a visit in her dorm, and gives her an ultimatum - if she makes an effort and joins a group at College , but still doesn't like it by the end of the year, she can quit and follow her true dream of becoming a DJ, and he will help her... but he wants her to really make an effort.
As Beca goes into the communal showers singing to herself, Chloe happens to be in the showers as well, and confronts Beca in her shower stall , insisting that she try out for the Bellas. Although Beca is mortified, she sings along with Chloe to 'Titanium'.
Finally the auditions are on, and Jesse, Benji and Fat Amy are among the hopefuls trying out, auditioning to 'Since you've been gone'. Beca comes late, and hasn't prepared the audition song, but auditions with 'Cups (miss me when I'm gone)', and Chloe and Jesse are impressed, while Aubrey is unsure. We then see the initiation of the new Bellas including Beca, Fat Amy, Stacie (Alexis Knapp), Cynthia-Rose (Esther Dean) and the very softly spoken Lilly (Hana Mae Lee). They all swear an oath and drink the 'blood of the Bellas who came before them' (wine). Cut to the initiation of the new Trebles.. Jesse has made it, but Benji has not. That night, there is Aca-initiation where all the groups come together for drinks and singing. Jesse is pleased to see Beca is in the Bellas, although she seems less than thrilled to be there. Fat Amy and Bumper seem to share some chemistry, despite insulting one another, and Aubrey looks unhappy that Beca is showing an interest in Jesse.
The next day, Bella rehearsals begin. Aubrey has a very rigorous training schedule planned. She announces that one of the new Bellas has been kicked out for breaking the oath that you must not hook up with a Treble. She asks the rest of the girls to confess the same if necessary, and another Bella is kicked out. Rehearsals begin, and the girls are less than impressive. Aubrey and Chloe have their work cut out for them. As the girls leave, Aubrey calls Beca over and warns her not to get involved with Jesse. Beca is flippant and retorts that Aubrey is not the boss of her.
At the Bellas' first gig with their new line up they wear the same uniforms and sing the same songs they always have. They are not at all prepared, and are cut short and told they won't be paid for the gig. Aubrey is furious, especially at Chloe, who then reveals she has Nodes - 'the rubbing together of your vocal chords at above average rates and without proper lubrication', and will have to pull back on her singing.
As time goes on, Jesse tries flirting with Beca at work, who seems like she is interested in him but trying not to be. He then approaches her on campus and wants to show her his favourite movies, but she reveals she does not like watching movies, especially the endings... Jesse argues that endings are the best part. Beca says she doesn't have time to watch the movies, as Bellas are rehearsing all the time. Jesse asks if they are getting ready for the Riff Off. Beca has no idea what that is, but soon finds out. The Riff Off is a singing competition that pits the groups against each other, with various song categories. Despite Beca impressing most of the Bellas, the other groups, and Jesse with her version of 'No Diggity', the Trebles win the Riff Off. Beca is encouraged by the other Bellas' ability to listen to one another and sing together impromptu, but Aubrey brushes her off, upset that they still lost.
Jesse and Beca become closer when she plays him some of her music remixes, and he shows her the ending of his favourite movie, the Breakfast Club. As the Bellas rehearse for the next round of competition, Chloe and Beca try to convince Aubrey to try something different and not keep singing the same songs, but Aubrey is adamant that they will win if they keep it all the same. She gives Fat Amy a solo in the performance, who gets a bit carried away, but the audience love the energy and variation from the usual Bella prim and proper style. However, Aubrey is not impressed. The Trebles win the trophy, but as they are all leaving, a fight breaks out with an acapella group of older men outside, and Beca accidentally breaks a window, and ends up being arrested. When her father arrives to bail her out, she is furious with Jesse for calling him. Beca arrives back at the dorm, and all the Bellas are waiting for her in support, although when she tries suggesting they mix things up for their next performance, once again, Aubrey shoots her down and says they will be doing things the same as always.
The Bellas travel to the semi finals on a bus driven by Fat Amy. She stops at a gas station to 'pump and dump', and the Trebles drive past. Bumper cruelly throws a huge burrito at Fat Amy and sauce spills all over her, making her furious. As the Bellas keep driving, they are in good spirits and sing together 'Party in the USA', until suddenly the bus splutters, and Fat Amy realises she never actually got around to putting gas in the tank. Out of desperation, they call the Trebles for a lift on their bus. At the semi finals, there is a group called the 'Footnotes' who are impressive, but the commentators note the male lead singer's voice sounds unusually high. The Bellas begin to perform their usual set, but Beca becomes sick of the mocking looks from the other bands, and also the bored looks in the audience, so she starts singing another song 'Bulletproof' over the top of 'The Sign' much to the other Bellas' surprise and Audrey's displeasure, but it does make the judges take more notice. After the performance, Aubrey tells Beca off, and she ends up leaving the Bellas, but not before lashing out at Jesse again as he tries to come to her defence.
The Bellas do not advance to the Finals, and Aubrey is devastated. Over Spring Break, Beca finally watches the Breakfast Club, and is moved to tears. Also, it is discovered that the group the Footnotes who advanced to the Finals had a member in High School, so were disqualified, and the Bellas are back in the running. They are all thrilled and excitedly come together to rehearse. Chloe reveals that she has let Beca know, much to Aubrey's annoyance. Beca tries to make amends with Jesse, but he brushes her off. She speaks to her Dad, admitting that she shuts people out, and asks him what to do. Meanwhile, Bumper has been offered a back up singer job with John Mayer,and ditches the Trebles without any regard for the Finals. Jesse immediately rushes to Benji and offers him the spot, but warns him not to act weird.
Finally sick of Aubrey and her bossy, controlling attitude, Chloe tells Aubrey that Beca was right, and they should have listened to her. They get in a fight and the other Bellas start to intervene. Aubrey loses it, and makes herself projectile vomit all over the place. All hell breaks loose, as Beca comes in and calms everyone down. Finally Aubrey agrees to let Beca have a go at remixing the songs and changing the style of the Bellas' performance for the better. It is discovered that as Chloe has had surgery to remove her Nodes, she can now hit the bass notes, and also that Lilly can beatbox.
It is the Finals, and Beca wishes Jesse luck in his performance, but there is still tension between them. The Trebles perform strongly as always, with Benji finally getting his moment to shine with a solo. It is then the Bellas' turn to perform, and they have ditched their drab flight attendant uniforms and are wearing their own clothes and look a lot more stylish and sexy. They do a fantastic performance with all different songs mixed together, met with thunderous applause from the audience including a standing ovation from the Trebles. Jesse finally realises Beca is sorry for how she's treated him as one of the songs she sings is 'Don't you forget about me' from the Breakfast Club soundtrack. After the performance, Beca and Jesse finally share a kiss. Cut to 6 months later, and it is next years' auditions. The winners of last year's championships get to pick the audition song, and we see the Bellas minus Chloe and Aubrey with a huge trophy on their table discussing which song to pick.