The young female orphans in the government-run Prescott Academy are trained to become field operatives. Their number one rule taught by trainer Hardman is “no attachments”. Agent 83’s top skills are rivaled by Agent 84. 83 is curious about the world, learning teen culture through magazines and watching Mean Girls and Beverly Hills, 90210.

Hardman assigns the orphans to capture Victoria Knox, an arms dealer. Undercover in Chechnya, 83 poses as one of those captured and brought before her. She latches herself and Knox onto Hardman’s passing jet but 83 drops into a river as Knox is apprehended, causing Hardman to declare her missing. 83 adopts the name Megan Walsh, poses as a Canadian student, and arrives in Newtown, Connecticut to live with her host family, the Larsons. Mrs. Larson and her son, Parker, welcome Megan, but Liz, her daughter, is cold and distant. Megan is mocked at school, leaving both her and Liz embarrassed. She soon makes friends with Roger Marcus, a classmate, and becomes attracted to Cash Fenton, a popular student, hacking the school’s system to assign him as her biology lab partner.

Cindy and Donna, popular bullies, convince Megan to try out as the school mascot. When students from a rival school follow tradition and try to kidnap Newton High’s mascot from a game, she fights them off, with a video of her actions going viral. It earns her some popularity while annoying Liz. Later, Megan is apprehended by a Prescott agent and brought to Hardman. He assumes she is working with their enemies, but she reveals she wanted to enjoy normal life outside the academy, leading to Hardman letting her go with a warning. Megan and Liz both attend a house party hosted by the school’s class clown, Gooch. Megan hangs out with Cash, while Liz gets drunk and begins to bond with Gooch and learns that his real name is Benward, as 84 arrives at the party, calling herself Heather. Megan assumes Hardman sent Heather to shadow her, but Heather denies this and tries to seduce Cash to irritate her. Megan eventually wins Cash’s attention and gets asked by him to the homecoming dance.

The next morning, Hardman warns Megan that Knox has escaped from the academy. Liz drives her to school, but they are pursued by a masked assassin. Megan reveals her secret to her and crashes the two cars, but the assassin escapes. Megan recognizes the smell of perfume, identifying Heather. The two leave for the hospital, where Roger and Gooch visit the girls separately. At the homecoming dance, Megan becomes bored by Cash and leaves him before making amends with Roger. However, Heather reveals herself as Roger’s date, provoking a fight between her and Megan. She further discloses she joined Knox’s operation in order to get a chance to kill Megan. Liz sneaks up on Heather and stabs her in the leg and the two escape.

Megan and Liz return home to find that Knox and her mercenaries have seized Mrs. Larson and Parker as hostages. Knox reveals that she was Prescott Agent 1, but she left and turned against them for robbing her of her life. Hardman comes in with reinforcements and subdues Knox and her mercenaries. Megan uses a Prescott helicopter to stop Roger, who is driving home. She tells Roger her true feelings for him and the two kiss before joining Liz and Gooch in the helicopter.

In a mid-credits scene, Heather meets with a former henchman of Knox and orders him to find out where Megan is going to college.