Three years after her family’s death in a plane crash, Stephanie Patrick has become a drug-addicted sex worker in London. She is approached by journalist Keith Proctor, who believes the plane crash was a terrorist attack covered up by the government. Escaping her brothel, Stephanie starts living with Proctor and studies his research on the crash, which he explains was caused by a bomb made by a man named Reza, who attends university in London. Stephanie buys a gun and finds Reza, but cannot bring herself to shoot him and is humiliated when he indirectly mocks her and leaves. Hours later, she returns to Proctor’s apartment and finds him murdered.
Through Proctor’s notes, she discovers his confidential source is “B”, a disgraced MI6 agent named Iain Boyd. She travels to Scotland and finds Boyd living in seclusion; after she explains that she has nothing to lose and wants revenge, he reluctantly agrees to train her to hunt down Reza. Boyd explains that Reza was hired by a terrorist known only as U-17, who downed the plane to kill liberal Muslim reformer Abdul Kaif; Kaif’s father Suleman funded Proctor’s investigation into the crash. Stephanie trains for months to assume the identity of Petra Reuter, an assassin killed by Boyd whose body was never found.
As Petra, Boyd sends her to Madrid to find Marc Serra, an ex-CIA agent turned information broker, who may lead her to U-17. Stephanie asks Suleman to finance her mission; he refuses, but Kaif’s mother Alia offers her the money. Stephanie tracks down Reza and leaves him to die in a bus bombing. Alia thanks her and returns her father’s ring. Realizing that Serra is U-17 since he has been asking Stephanie to kill all known connections to U17, Stephanie kills him with a syringe at his home. Two weeks later, Boyd confronts Stephanie in London, revealing that he will be allowed back into MI6 if he eliminates “Petra”. Warned to disappear, Stephanie walks away having finally found peace.