Three years after her family's death in a plane crash, Stephanie Patrick works as a prostitute in London and is addicted to drugs. One day, she is approached at her brothel by a journalist, Keith Proctor, who tells her that the plane crash was not an accident, but was a terrorist attack covered up by the government.
Though she initially doesn't believe him, Stephanie visits his apartment, where she sees all of his research on the crash. He tells her that the plane crash was caused by a bomb made by a man named Reza, who attends university in London. Stephanie buys a gun and tracks Reza to the university cafeteria. She plans to kill him, but at the last second, she is unable to pull the trigger and Reza escapes. Hours later, Stephanie returns to Proctor's apartment, where she had been staying, and finds Proctor dead.
Through Proctor's notes, she discovers that his source for his research, B (an ex-MI6 agent), lives in a remote location in Scotland. She travels to Scotland to find B, who is disgusted with her quest to find and kill Reza. After she explains that she has nothing to lose, he reluctantly trains her to complete her quest. B, Iain Boyd, explains to her that Reza was hired by a radical terrorist, U-17, who downed the plane to kill liberal Muslim reformer Abdul Kaif. The rest of the plane crash was collateral damage. Stephanie is trained to assume the identity of an assassin, Petra Reuter, whom Boyd killed, but whose body was never found.
After months of extensive training, Boyd instructs her to go to Madrid to find Marc Serra, an information broker and ex-CIA agent. Serra tells her that he needs Lehmans killed, a notorious gangster in Tangier, who arranged for the bomb to be on the flight. Boyd tells her to take the job, to prove herself as Petra Reuter. She travels to Tangier and finds Lehmans, who requires an oxygen mask to breathe. After a prolonged fight, Lehmans dies when separated for his breathing mask for too long. Stephanie is discovered by Lehmans' men, but escapes after a car chase.
Serra next tells Stephanie to kill businessman Leon Giler in New York. Boyd once again tells her to take the job, as Giler financed the bombing and U-17. Boyd goes with her to New York and provides her with a knockout gas inhaler, and a knife to slit his throat. Stephanie poses as a prostitute and seduces Giler, but is unable to kill him and leaves. As Giler leaves the hotel and gets into his car, Boyd detonates a remote bomb, killing Giler, along with his chauffeur and two young children.
Back at their New York hotel, Stephanie is horrified by the death of Giler's children. Though Boyd had only planned to kill Giler, he is unapologetic. He also reveals that he was released from MI6 after killing Petra, who killed his wife. The CIA disapproved of killing Petra, as she had intel on U-17; thus Boyd also feels responsible for the plane bomb. With this new information, Stephanie leaves Boyd, and returns to Madrid to be with Serra, with whom she begins a relationship.
Boyd warns her that U-17 hired Serra to find someone to eliminate Giler and is eliminating all ties to him, which leads Serra to suggest that Reza is U-17. Serra provides Stephanie information that Reza is in Marseilles. She discovers him and his associate preparing for another bombing. Following them onto a bus, she attacks Reza. Able to evacuate, she fights and kills Reza, escaping the bus before the bomb is detonated.
Stephanie returns to Serra's house and reveals that she is not Petra Reuter. She kills him with a deadly venom, revealing that she has discovered that Serra is U-17, as Reza was too inconsequential to be the mastermind behind the bombing. Her revenge complete, she leaves his house and visits Proctor's parents, whom she met earlier to express condolences for his death and to promise his death will not be in vain.
- Blake Lively as Stephanie Patrick
- Jude Law as Iain Boyd
- Sterling K. Brown as Marc Serra
- Max Casella as Leon Giler
- Daniel Mays as Dean West
- Geoff Bell as Green
- Richard Brake as Lehmans
- Raza Jaffrey as Keith Proctor
- Tawfeek Barhom as Reza
- David Duggan as David Patrick