Brandy Klark (Aubrey Plaza), from Boise, Idaho, is an overachieving but socially awkward teenager who graduates as the valedictorian of her high school in 1993. After the ceremony, Brandy’s two best friends, Wendy (Sarah Steele) and Fiona (Alia Shawkat), take Brandy to a party, where she gets drunk for the first time. Brandy makes out with a muscular college boy she has a crush on named Rusty Waters (Scott Porter). Since the room is dark, he mistakes Brandy for someone else and when he realizes who she is, he rejects her. Brandy blames her lack of sexual experience and resolves to learn all about sex over the summer to prepare for college. She decides that her end-of-summer goal will be to have sex with Rusty to complete her “To Do List”.
Brandy gets a job at the pool as a lifeguard to be close to Rusty and her study-buddy Cameron Mitchell (Johnny Simmons). As the newbie, she is hazed by her slacker boss, Willy (Bill Hader) and her other co-workers by being given the most unpleasant jobs. Brandy is told to clean waste from the pool that appears to be feces. She assumes her co-workers are playing a prank on her, based on the Baby Ruth joke from the film Caddyshack, so she takes a bite only to find out it is actual feces. As revenge, she pushes Willy, who does not know how to swim, into the pool. She agrees to teach him how to swim in exchange for ending the hazing.
Brandy gets advice from her sister, Amber (Rachel Bilson), her mother, and her two best friends, while her father, a conservative judge, is uncomfortable with the talk of sex. Using this information, she makes a “to do list” of sexual acts to learn and perform. As the summer progresses, Brandy has several sexual encounters with Cameron and other boys, all while trying to catch Rusty’s eye. Cameron begins to fall in love with her but is crushed after discovering her list and realizing he was just part of her “mission”. Willy catches Brandy, Wendy, Fiona, and adult members of a male grunge band in the pool after hours. Brandy is sent home where she is confronted by Cameron over the list. Cameron leaves in a huff and Brandy cries. Duffy (Christopher Mintz-Plasse), whom Wendy has a crush on, comes over to comfort Brandy and the two hookup.
When Wendy and Fiona come over to watch the film Beaches with Brandy, they discover her list and see the list of boys with whom she had experimented. They get upset after finding out that Brandy hooked up with one of their crushes and leave angrily. They declare that Brandy has failed to put “Bros before hoes” and call her a slut.
Brandy finally gets close to Rusty when they vandalize a rival pool, but they get caught because Brandy leaves her bra, which has her name on it, and Willy fires her. Brandy asks out Rusty, and he takes her to a popular make-out spot to have sex, but the sex is brief and disappointing. She sees her father and mother in the Dodge Caravan next to them having sex, causing her to freak out and demand that Rusty take her home immediately.
Willy goes to the Klark house to stop Brandy from having sex with Rusty, but is met at the door by Amber, who seduces him. When Rusty and Brandy arrive home, a jealous Cameron is there to meet him with a sucker-punch, and they fight until Brandy breaks it up. She compliments them on their good qualities, apologizes sincerely to Cameron for using him, and offers her own view of sex.
Afterward, she seeks out Wendy and Fiona to apologize to them. She sings “Wind Beneath My Wings” at Wendy’s door, and the two girls join in, forgiving her. Brandy meets with Willy at the pool, and he offers her his job if she comes back next summer as he has quit to follow the Grateful Dead.
In the fall, Brandy and Cameron meet again at Georgetown University. Brandy apologizes to Cameron. They have various forms of sex and Brandy finally achieves orgasm, the last thing on her list, just as her father walks in on them.