The girls at Prescott Academy are groomed from a young age to become elite assassins, instilled with the primary rule of "no attachments". However, Number 83, portrayed by Hailee Steinfeld, has always been captivated by the world outside her training grounds. Despite excelling in her assignments, she longs for a life filled with normalcy. Her mentor, Hardman, played by Samuel L. Jackson, consistently warns her and her peers against emotional connections, fostering a tense rivalry between her and Number 84, played by Sophie Turner.
When Hardman tasks the girls with capturing the dangerous arms dealer Victoria Knox, played by Jessica Alba, they venture to Chechnya for the mission. In a daring undercover operation, 83 manages to get close to Knox, even escaping with her onto an arriving jet alongside Hardman. However, as chaos ensues, 83 seizes the moment to leap from the jet into a river, leaving the agents to discover her absence and declare her Missing in Action (MIA).
Choosing to abandon her assassin life, 83 embraces her newfound identity as a regular teenager. She immerses herself in teenage culture, scouring through glossy magazines and watching films like Mean Girls and Bring It On, before enrolling in high school under the alias Megan Walsh. Welcomed by the Larson family, which includes Mrs. Larson, portrayed by Rachael Harris, and her children, Liz, played by Dove Cameron, and Parker, portrayed by Jason Ian Drucker, Megan finds herself quickly navigating the ups and downs of adolescence. While Parker shows interest in her, Liz remains distant.
On her first day at school, Megan’s attempt to make an impression through her unique outfit backfires, leading to ridicule. However, Liz helps her out by lending her better clothes. During a school assembly, Megan introduces herself as being from Regina, Canada, but her pronunciation leads to laughter among her peers, which is only alleviated when the charming Cash, played by Toby Sebastian, performs on stage.
As Megan adapts to high school, she befriends a socially awkward classmate named Roger, played by Thomas Mann, who harbors feelings for her, despite her crush on Cash. After deliberately pairing with Roger for a biology project, they face hostility from two of their school’s mean girls. When they challenge her to showcase more school spirit, Megan utilizes her assassin skills to secure a spot as the school mascot, inadvertently gaining popularity—though it ruffles Liz's feathers.
While Hardman maintains control over Knox at Prescott, Megan's life at school takes a dramatic turn. She grows closer to Cash, leading to jealousy from Roger. Just as friendships blossom, Megan is confronted by a Prescott analyst, who drags her back to Hardman, where a mistaken assumption leads to a moment of vulnerability as Megan reveals she merely wished for a life beyond the Academy.
With a night out at a party beckoning, Mrs. Larson encourages Megan to attend and makes Liz join her. The party introduces new dynamics, especially as Megan witnesses a rivalry brewing with 84, now known as Heather. Believing Hardman has sent Heather to shadow her, conflict ignites during a game, culminating in Megan winning Cash’s affection by being asked to the homecoming dance.
However, danger looms as Hardman warns Megan of Knox's escape and she is pursued by a masked assassin. A dramatic confrontation ensues where both the girls face life-threatening scenarios. Determined not to put the Larson family at risk, Megan prepares to sacrifice her happiness for their safety, but Liz counters that abandoning them would cause more harm.
At the homecoming dance, when Megan realizes Cash is not the thrilling match she envisioned, she breaks it off and seeks Roger’s forgiveness, discovering he is with Heather. Fighting erupts between the girls, leading to a chaotic scene involving punch spills and unexpected alliances. Liz ultimately comes to her sister's aid, subduing Heather long enough for Megan to escape home, only to find Knox has taken her family hostage.
In a thrilling climax, Megan exhibits her training once more, combating Knox's associates despite the personal stakes. Ultimately, Hardman arrives, demonstrating a surprising commitment to Megan's safety. As the dust settles, the true implications of life outside Prescott become clear for Megan and those she has grown to love, hinting at an uncertain yet hopeful future as she receives unexpected support from Hardman, suggesting her journey may just be beginning.
