NOTE: This spoiler was submitted by Jeremy

The film opens with an assassin named Sam (Karen Gillan) saying she works for a shady organization called The Firm. When they need to have a target taken care of, they send Sam. She is seen going up against a whole gang of men all by herself before returning to her apartment injured. She is contacted by her handler, Nathan (Paul Giamatti), who asks to see her at the diner.

Sam goes to the diner, one that she has been going to for years. A flashback from 15 years earlier shows Sam (here played by Freya Allen) waiting for her mother Scarlet (Lena Headey) for three hours to pick her up. When Scarlet arrives, she apologizes to her daughter and they share a milkshake after Sam gives her a gun stashed in a book. Moments later, a gang of Russian goons come for Scarlet, so she hides. The leader approaches Sam and threateningly asks her for her mom. Scarlet comes out and executes all the men in the diner before having to flee the place. Sam tries to run after her, but Nathan grabs Sam while Scarlet tearfully looks back and drives away.

Nathan shows up to meet Sam, chastising her for making a mess of a job despite him having told her she would only have to deal without about four or five guys and not the whole gang that went after her. He gives Sam her next assignment, which is to retrieve money from a man that has stolen from The Firm.

At the scene of Sam’s last hit is Jim McAlester (Ralph Ineson), leader of an Irish gang. Apparently, his son was among the casualties of Sam’s job, and he orders his nephew Virgil (Adam Nagaitis) to find those responsible for his son’s death.

Sam visits a library that is a front for an organization of assassins that Scarlet used to be part of. She meets Madeleine (Carla Gugino), Florence (Michelle Yeoh), and Anna May (Angela Bassett). They don’t fully recognize Sam until she tells them she is Scarlet’s daughter. They take her old weapons, some of which she stole from there, and they give her clean guns stowed away in classic novels.

Sam goes to the man’s apartment and finds the money in a panda-shaped suitcase. The man, David (Samuel Anderson), finds her, but Sam aims her gun at him. His phone starts ringing and he insists he has to take it, but Sam tells him not to move. David goes for the phone, and in the struggle, Sam accidentally shoots David in the stomach. He still manages to answer the phone, revealing that some men have kidnapped his daughter Emily (Chloe Coleman) and the money was for her safe return. David pleads with Sam to give the money just for Emily’s safety. Sam takes pity on him and brings him to a special hospital for assassins to try and patch David up while she completes his task.

Sam heads to the bowling alley where David was instructed to go. Nathan calls her and she tells him that she is going to save Emily. He orders her against it and to bring the money back to The Firm, but she disobeys him. Nathan then sends his three goons, the Boneheads – Crow (Jack Bandeira), Shocker David Burnell IV), and Yankee (Ivan Kaye) – to go after Sam but to not harm her. Sam arrives at the bowling alley and follows the instructions of the leader. As Sam changes clothes per his demands, the Boneheads show up and order Sam to give them the money. Sam once again refuses and fights the Boneheads, which the kidnappers witness. They begin to get rougher with her, but Sam incapacitates all of them before getting the money back. Meanwhile, Nathan learns that Sam killed McAlester’s son, and he contacts McAlester to throw Sam under the bus and tell his men to go after her.

Sam meets the kidnappers at a video store where they give her Emily in return for the money. After making sure Emily is safe, Sam goes after the kidnappers, when one of the men turns on his crew and shoots the driver in the head. The car crashes and another one dies before the traitor tries to escape. The leader goes after him and shoots the traitor just as he pulls out a grenade, blowing up himself AND the money. Sam then kills the leader with the broken end of a mop after he tries to shoot her.

Sam brings Emily to the hospital where they find that David has died. The Boneheads show up at the hospital, high off laughing gas, as the doctor (Michael Smiley) tells them Sam is there and they can use a paralyzing serum on her and beat her up. The doctor injects Sam in the arm but is pushed out of the room as Sam tells Emily what to do. The girl tapes a gun and knife to Sam’s hands while she has use of her fingers and proceeds to take on the Boneheads. She shoots Crow dead and slashes Shocker in the throat before getting Yankee to shoot him with a shotgun, and Sam finally crushes Yankee’s head with a giant tooth. Sam and Emily take the elevator down, where Sam prepares to execute the doctor, but at Emily’s behest, she changes her mind and reminds the doctor that a child helped spare his life.

Sam and Emily make their way to the parking lot where Sam gives Emily a driving lesson. This is as some of McAlester’s goons come for the two of them. In order to spare Emily from the sight of violence, Sam has her close her eyes as she guides her through the parking lot to avoid the goons after them. Together, the two manage to crush, behead, and impale several thugs before making their escape.

Sam brings Emily back to her apartment, only to be found by Scarlet. The two have a tense reunion, where Scarlet has to explain that she had to go on the run after she killed the man responsible for the death of Sam’s father, because he worked with The Firm and it made Scarlet a wanted woman, so she had to leave Sam behind for her protection. Soon, more McAlester goons come hunting for the women, so Scarlet manages to find a way for her, Sam, and Emily to escape.

The trio head to the library where they reunite with Scarlet’s old assassin sisters. Although Madeleine and Florence are happy to see Scarlet again, Anna May is none too happy due to her disappearing without so much as a heads-up. The thugs, led by Virgil, arrive at the library for a showdown. Emily wants to stay to help Sam, seeing herself as Sam’s apprentice, but in order to get Emily out of there, Sam confesses to her that she was assigned to kill Emily’s father and that she carried it out. The thugs break through while Sam and Scarlet stay behind for a shootout. Madeleine keeps Emily safe in a van while she and Florence get in on the action. The ladies battle the thugs and execute most of them while Virgil battles Madeleine with her sledgehammer. After a lengthy fight with the ladies coming out victorious, they find Madeleine mortally wounded, telling them that Virgil kidnapped Emily. She dies alongside her sisters, while Sam goes after Emily alone when Scarlet deems it too unsafe to do so.

Sam meets McAlester and his goons at the diner to get Emily back. McAlester meets Sam and tells him how he was so proud to have a son after having many daughters, so it is extra personal to him that Sam killed his son. She is genuinely remorseful for having done so and accepts whatever torture McAlester plans to inflict upon her. She is saved when Scarlet, Florence, and Anna May come by to protect her. Sam gets Emily out of the diner, and although the men are instructed to stand down, McAlester attempts to attack first with a knife, only to get a bullet to the head courtesy of Scarlet. She and her sisters proceed to gun down Virgil and the rest of the men.

Sam brings Emily to a hideout off the grid for protection. She has a nightmare that Emily kills her in retribution for her father’s death, so she tells her she is sorry for what happened with her father. Emily forgives her because Sam still tried to save him.

In the aftermath, Sam has Emily go to Nathan’s house posing as a girl scout to send a warning to him. Knowing that The Firm wants Sam and Emily dead, she uses a sniper rifle to get Nathan to send a warning to The Firm, stating that Sam, Scarlet, and the other Librarians have what it takes to kill all of them without any of them seeing it coming. Nathan gets the message.

The film closes with the women and Emily driving away to an unknown destination, but freer and closer than ever.

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Sam is an assassin for a shady organization called The Firm. While on a mission, she deviates from the task by rescuing the daughter of her target, an eight-year-old girl named Emily. After accidentally shooting and killing Emily's father, Sam takes it upon herself to protect Emily from her captors, as well as The Firm since her father stole money from them. In the process, Sam is reunited with her mother Scarlet, another assassin who previously abandoned her 15 years earlier after killing the people responsible for the murder of Sam's father.

Sam's handler Nathan sells Sam out to Jim McAlester, an Irish mob boss whose son was killed during an assignment that Sam carried out. Sam and Scarlet reunite with Madeleine, Florence, and Anna May, other assassins/"Librarians" that worked with Scarlet before she disappeared. McAlester's nephew Virgil leads the goons to attack the women after Sam admits to Emily that she killed her father. Emily is kidnapped by Virgil after the women kill McAlester's goons, and Madeleine is fatally wounded in the fight.

Scarlet and her assassin sisters help Sam rescue Emily and kill McAlester, Virgil, and all their goons. They even send a message to Nathan to have The Firm back off of them or else Sam and Scarlet will kill all of them. They adopt Emily, and she forgives Sam for what she did, joining the ladies on a journey to a new life.