NOTE: This spoiler was submitted by Mike

Stephanie Patrick (Blake Lively) is about to assassinate a man, but hesitates. We cut back to eight months previous, when Stephanie is working as a prostitute in a brothel. She is visited by Proctor, a journalist, who has come in the guise of a john. He reveals himself to her, letting her know that he knows her identity. Her family (her parents and siblings) died in the explosion of a trans-Atlantic flight. The wealthy father of one of the victims of the plane explosion has hired him to investigate, and he has found evidence that proves that it was not an accident, as widely thought, but an act of terrorism. After asking for a cigarette, Stephanie leaves the room. A burly man busts in soon after and roughs up Proctor before throwing him out. Stephanie uses drugs to blot out memories of family. Eventually, she takes a card that Proctor left behind before he was thrown out, and she runs away from the brothel. She calls him, and he picks her up.

At Proctor’s place, Stephanie finds a room filled with Proctor’s research into the explosion, including a wall full of photographs of the passengers. Pictures of her family are up on the wall, as is the picture of a man who took her spot on the plane when she decided not to go along on the trip. Proctor tells her that the man had a wife and children. He tells her some of what he’s learned: A young man named Reza built the bomb that destroyed the plane, and he is now a university student in London. Proctor believes that the intelligence services know about him, but for some reason, they are letting him roam free, perhaps due to a fear that arresting him might threaten an investigation into a larger network. Proctor reveals that he believes the purpose of the bombing was to assassinate an Islamic reformer named Abdul Kaif, and that all the other passengers were collateral damage. It is Kaif’s father who is bankrolling Proctor. He says that he has a contact who has helped him gather intelligence. While going through Proctor’s paperwork, Stephanie finds a folder marked “B,” and guesses that B is Proctor’s contact.

Proctor leaves Stephanie alone in the apartment. Before leaving, he catches her with his wallet. He leaves her some money and the keys to the apartment. She goes to a drug dealer operating out of a shop; he gives her some drugs, but she also wants a gun. She goes to a university cafeteria and finds Reza sitting with some friends. She sits at their table and surreptitiously takes out the gun, holding it out of sight below the table. Reza notices her watching him and asks if he can help her. She hesitates, and he and his friends leave the table. Stephanie puts the gun back in her pocket before noticing that a bag containing some of Reza’s files, which she had set down, is gone. She wanders around for a while before going back to the apartment. The place has been ransacked, and Proctor is dead. Not knowing what to do, Stephanie decides to track down B.

Stephanie gets to B’s place, a cabin in the middle of nowhere. She sneaks around the place, but he surprises her. B (Jude Law) disarms her of the gun she’d brought, and forces her into another building on the grounds, making her empty her pockets before locking her in. Later, he wakes her up and questions her, letting her know that he knows Proctor is dead and that she is related to victims of the explosion. He leaves her alone again. Stephanie suffers from withdrawal. During one conversation B asks why she was working as a prostitute; she says that’s the life she wants, but he questions if this could be true of someone who was once a promising university student. Another day, B give her some boots and takes her running. Thus begins her training, which expands to include shooting lessons. He tells her that the key to shooting well is getting your “rhythm section” (i.e. heartbeat and breathing) under control. He also shows her how to fight. They plan for her to assume the identity of Petra, an assassin who B killed when he was a member of MI6, and to track down members of the terrorist ring that perpetrated the explosion. Reza was the bomb-maker, but B believes Reza was working for an unidentified individual known by the CIA codename U-17. They are going to need funds for this endeavor, so Stephanie goes to see Suleman Kaif, the man who was funding Proctor. She sneaks into his house to see him, and explains what she wants. Assuming that she is trying to scam him, he threatens to throw her out, but his wife stops him, agreeing to give Stephanie money. Stephanie leaves her father’s wedding ring (her father having left it behind by accident before the flight), telling the woman to keep it for her.

Stephanie, now using the guise of Petra, goes on her first mission, to kill a man named Lehmans, who B tells her helped get the bomb on the plane. First she must see Serra (Sterling K. Brown), a man who supplies intel. She agrees to pay him $200 thousand for info on where to find Lehmans. Serra says he heard Petra was dead, but Stephanie keeps up the ruse. It turns out that Lehmans is the man who we saw Stephanie hesitating to shoot at the beginning of the film. Lehmans needs oxygen and appears frail. He asks who she is, and she says Petra, but he says that if she were really Petra he’d already be dead. When Stephanie hesitates, he attacks her, injuring her hand. He tried to inject her with a syringe, but he suddenly needs oxygen and abandons the struggle to try to get to his oxygen mask. Stephanie holds him back, and he dies. As Stephanie leaves the building, a car pulls up. A man rushes into the building as she sneaks out, but another man outside notices her and demands that she stop. She threatens him with a gun and steals his car. The men shoot at her, and get another car, chasing her through narrow, crowded streets. She manages to avoid being hit by an oncoming truck, and the truck hits the car chasing her, presumably killing the men. Later, she studies the syringe that Lehmans was trying to stick her with, finding that it is a deadly toxin.

She sees Serra again, angry that he didn’t warn her about the guards. He says that he assumed “Petra” would know that the man would be guarded. He tells her that a client wants a New York businessman named Giler killed. She tells him she’ll think about it. Later, B tells her that Giler is suspected of helping the terrorists, and that she should take the job, but to ask for a large payday because that’s what the real Petra would do. Seeing Serra again, she asks for $2 million, promising to pay him the $200 thousand she still owes out of the money. In New York, Stephanie and B go over the plan: Giler has a penthouse where he receives visits from prostitutes, and Stephanie will go under the guise of one of them. Since she will be searched before being allowed up, her only weapons are an asthma inhaler filled with a spray to help debilitate the man, and a knife disguised as a hairbrush. She goes up to Giler’s penthouse. She is about to slit the man’s throat when he begs for his life, telling her that he has a young daughter. She administers a shallow cut to his neck and escapes. B picks her up, and they see Giler rushing out into an SUV before the SUV drives off. Stephanie says that she couldn’t kill him. The SUV explodes.

A later news report reveals that Giler, his driver, and his two young children were all in the vehicle. It is revealed that B placed an explosive device on the vehicle as a back-up plan in case Stephanie failed her mission, but he insists that he didn’t know that kids would be in the car. It is also revealed that B was pushed out of MI6 after killing the real Petra against the CIA’s wishes. The CIA had hired her to track down and kill the mysterious U-17, but B killed her because she’d assassinated his wife, who was also an MI6 agent. Stephanie realizes that if B had not killed Petra, she would have assassinated U-17, and the plane bombing would not have happened. Angry at everything, she leaves him.

She goes back to Serra. He reveals that he used to work for the CIA, but left after he was asked to do things that bothered him. He got into his new line of work because he needed to, but it’s “just something he does now.” Serra learns the identity of U-17: it’s Reza, the bomb-maker, who everyone thought only worked for U-17. There is another terrorist act being planned, the bombing of an anti-terror march, and Stephanie has to stop it.

Stephanie finds Reza’s place. She sees him strapping a vest to a young woman. She hides in the apartment, gun in hand, preparing to make her move, when another man enters the apartment. When the new man sees her, she shoots him. Reza and the woman escape after he and Stephanie exchange gunfire. She chases them onto a bus. Reza tries to get off the bus, but Stephanie stops him. He warns that if he dies, a bomb will go off, killing everyone. She takes hold of him and yells for everyone to get off the bus because there’s a bomb. As people struggle to get off, the young woman with the vest pushes the button to detonate the bomb, but it fails to explode. Stephanie shoots the woman, wounding her. Reza struggles with Stephanie, attempting to stab her. She manages to get the knife and stab him on the leg. The woman with the vest realizes that the bomb is on a timer, meaning Reza lied to her by leading her to believe she had control over when it would go off. The woman yells that Reza is a coward, and holds him back as he tries to prevent Stephanie from fleeing the bus. Stephanie manages to escape just before the bomb goes off, killing the only people still on the bus, Reza and the woman with the vest.

Stephanie goes to see Serra. They hug, and she injects him with the syringe that Lehmans tried using on her. He asks why, and she tells him that he made a mistake telling her that Reza was U-17, as it was clear that Reza was inconsequential, and not the leader. She realizes now that Serra is really U-17, and that he had her killing people who knew his identity. Serra dies.

Stephanie sees the Kaifs again, letting them know that she killed everyone involved in the plane bombing, though the public will never know because the official story is still that the bombing was an accident. Mrs. Kaif gives Stephanie her father’s ring back.

Later (it seems that some time has passed), Stephanie is walking down a street when B sneaks up on her and gets her up against a wall. She asks if he is back with MI6, and he says that he could be if he snapped her neck. She quickly produces a knife, showing that she has learned to take care of herself even against him. He lets her go, telling her that it’s time for “Petra” to be dead again. Stephanie agrees since she is using a new name anyway. She walks off.