A Christian musical procession waits with a symbolic block of tofu outside a prison for the release of Lee Geum-ja (Lee Young-ae), a reformed female prisoner. Convicted of kidnapping and murdering a 6-year-old schoolboy, Won-mo, 13 years earlier, Geum-ja became a national sensation because of her young age, angelic appearance, and eager confession to the crime. However, she became an inspirational model for prisoner reform during her incarceration, and her apparent spiritual transformation earned her an early release. Free, she is now intent on revenge.
Geum-ja quickly shows that her "kind-hearted" behavior in prison was a cover to earn favor and further her revenge plans. She visits the other paroled inmates, calling in favors that include food, shelter, and weapons. She begins work in a pastry shop and starts an affair with a young shop assistant, Geun-Shik, who would be the same age as Won-mo, had he lived.
It is revealed that Geum-ja did not smother Won-mo. The detective on her case was aware of her innocence, but helped her fake crime-scene details to ensure her confession looked credible. As a young high school student, Geum-ja had become pregnant and, afraid to go home to her parents, turned to Mr. Baek (Choi Min-sik), a teacher from her school, for help. Mr. Baek expected Geum-ja to provide sex and assist in his kidnapping racket in return. He used her to lure 5-year-old Won-mo to him, with the intent of ransoming the child, but murdered the boy. He then kidnapped Geum-ja's infant daughter and threatened to murder the baby if Geum-ja did not take the blame. She has spent her time in prison planning revenge on Mr. Baek for the murder of Won-mo, causing Geum-ja's child to grow up without a mother, and for sending her to prison.
Geum-ja discovers that her daughter was adopted by Australian parents. Jenny, now an adolescent, does not speak Korean and does not initially embrace her mother, though she does return with Geum-ja to Seoul to bond. Geum-ja plans to kidnap and murder Mr. Baek, now a children's teacher at a preschool, with the aid of his wife, another ex-convict. Mr. Baek hires thugs to kill Geum-ja and Jenny but Geum-ja kills them and Mr. Baek is subdued.
Mr. Baek wakes up tied to a chair in an abandoned schoolhouse. On his cell phone strap, Geum-ja discovers the red marble from Won-mo's crime scene, which had been taken as a trophy, and is horrified to see other children's trinkets also on the strap. After shooting him in both feet, she discovers snuff tapes in his apartment of the other children he had murdered. He had not been part of a ransoming racket; he would kidnap and murder a child from each school he worked at because he found them annoying. After killing each one, he would fake a ransom call to the parents, collect the money, and move on to a different school.
Sickened that four more children died because Geum-ja did not turn in the real killer 13 years ago, Geum-ja and the original case detective contact the parents and relatives of the missing children to the school. After watching each tape, the group decides to murder Mr. Baek together. They take turns beating, mutilating and torturing him until he is dead, then take a group photo, ensuring that none of them can turn in the others without implicating themselves. They then bury the corpse outside.
Geum-ja, the detective, and the relatives all converge at Geum-ja's bakery. Afterward, she sees the ghost of the murdered child who then transforms into his grown self (the age that he would have been if he had lived). Later, she meets Jenny and instructs her daughter to live purely, like tofu. She buries her face in an all white frosting covered cake and weeps as Jenny hugs her.
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