A Christian musical procession gathers outside a prison, holding a symbolic block of tofu, awaiting the release of Lee Geum-ja (Lee Young-ae), a woman who has transformed during her time as a prisoner. Thirteen years ago, she was convicted of kidnapping and murdering a 5-year-old boy named Won-mo, a crime that made sensational headlines due to her youthful appearance, angelic demeanor, and her eager confession. However, during her incarceration, Geum-ja emerged as an inspiring figure for prison reform, ultimately earning her freedom through apparent spiritual redemption. Now free, she is driven by a fierce desire for vengeance.
Once released, Geum-ja's seemingly benevolent behavior in prison is revealed to have been merely a facade, orchestrated to gain favor and facilitate her clandestine retribution plans. She reconnects with other paroled inmates, calling on their assistance for essentials like food, shelter, and weaponry. Taking a job at a pastry shop, she begins an affair with Geun-Shik, a young assistant who serves as a poignant reminder of what Won-mo could have become had he survived.
In a shocking twist, Geum-ja is not the murderer of Won-mo, as she had been wrongfully accused. The detective who led her case knew of her innocence but conspired to stage the crime scene to make her confession believable. Geum-ja's descent into this nightmare began in her teenage years when an unexpected pregnancy led her to seek help from Mr. Baek (Choi Min-sik), a teacher who exploited her vulnerability. He sinisterly manipulated her into assisting with his kidnapping scheme, luring Won-mo to his fate, while he ultimately murdered the child and threatened to harm Geum-ja's newborn daughter if she spoke out. The emotional toll of these events has fueled Geum-ja's quest for revenge, as her child was robbed of a mother, and her life was unjustly altered by the brutal act.
In her pursuit, she learns that her daughter has been adopted by Australian parents. Now a teenager named Jenny, she initially struggles to connect with Geum-ja, especially since she does not speak Korean. Nonetheless, they make the journey back to Seoul together to begin to forge a bond. Geum-ja's revenge plot thickens as she prepares to confront Mr. Baek, who is now a teacher at a preschool, with the assistance of his wife, another ex-convict. However, Mr. Baek's apparent power allows him to send thugs to eliminate Geum-ja and Jenny. In a brutal turn of events, Geum-ja not only defends herself but also manages to subdue Baek.
When Mr. Baek regains consciousness, he finds himself bound in a derelict school. Geum-ja discovers an orange marble on his cell phone strap, a distressing reminder of Won-mo's case, alongside other grim trinkets belonging to different children. After inflicting punishment on him by shooting his feet, she uncovers disturbing snuff films in his home, revealing his grisly true nature as a predator who kidnapped and killed children just for his own sadistic amusement.
Realizing that her silence has led to the deaths of additional innocent children over the past thirteen years, Geum-ja and the original detective reach out to the families of the missing children, gathering them at the school. Together, they confront their shared grief and rage as they decide to take justice into their own hands. In a harrowing act of vengeance, they systematically brutalize and murder Mr. Baek, ensuring their complicity binds them in silence through a group photo, effectively sealing their collective secret. After burying his body, the group disperses.
In the aftermath, Geum-ja, the detective, and the bereaved families reconvene at her bakery. A haunting moment occurs when she sees the ghost of Won-mo, who transforms into the age he would have been had he lived. Later, in a tender moment with Jenny, Geum-ja imparts wisdom, encouraging her daughter to embrace a life of purity, much like tofu. In tears, she buries her face in an all-white frosting-covered cake, while Jenny embraces her lovingly, marking the bittersweet end of a long, painful journey.
