Three years after her family's tragic death in a plane crash, Stephanie Patrick, portrayed by Blake Lively, finds herself lost in a world of addiction and despair, becoming a drug-dependent sex worker in London. Her life takes an unexpected turn when journalist Keith Proctor, played by Raza Jaffrey, approaches her with a theory that the crash was no accident but a terrorist act hidden by the government.
Desperate for an escape from her bleak circumstances, Stephanie leaves her brothel and begins living with Proctor, immersing herself in his investigation into the incident. Proctor reveals that the catastrophe was orchestrated by a man named Reza, portrayed by Tawfeek Barhom, a university student in London who allegedly constructed the bomb. Fueled by rage and a thirst for revenge, she purchases a gun and confronts Reza. However, she falters when he taunts her, ultimately leaving her in a state of humiliation.
Returning to Proctor's apartment, Stephanie discovers her mentor brutally murdered. Sifting through his notes, she learns about his secret informant, a disgraced MI6 agent known as Iain Boyd, played by Jude Law. Determined to avenge her family, she seeks him out in seclusion in Scotland and expresses her willingness to risk everything for revenge. Impressed by her resolve, Boyd reluctantly agrees to train her to track down Reza.
As they delve deeper into the investigation, Boyd reveals that Reza was contracted by a shadowy terrorist known only as U-17, who orchestrated the attack to eliminate liberal Muslim reformer Abdul Kaif. The financial backer of Proctor's investigation was Kaif's father, Suleman, portrayed by Nasser Memarzia. Over several months, Stephanie undergoes rigorous training to assume the identity of Petra Reuter, an assassin whose body was never recovered after being killed by Boyd.
Under her new guise, Petra, she travels to Madrid to meet Marc Serra, an ex-CIA agent now serving as an information broker who could connect her to U-17. Although Suleman initially declines to fund her mission, Kaif's mother, Alia, played by Amira Ghazalla, steps in to support her financially. With a renewed purpose, Stephanie locates Reza and orchestrates his demise in a bus bombing. Alia expresses her gratitude, returning her father's ring as a token of their intertwined fate.
Realizing that Serra is U-17—having orchestrated the killings of anyone linked to U-17—Stephanie confronts him in a tense showdown, ultimately ending his life with a syringe. However, her actions draw the attention of Boyd, who tracks down Stephanie in London and reveals a shocking twist: he faces reintegration into MI6 if he can eliminate "Petra." In a crucial moment of decision, she chooses to vanish, having finally discovered a semblance of peace in her tumultuous journey.
