Vidya Sinha (Vidya Balan) is a middle-class working mother who lives in Chandan Nagar, West Bengal with her teenage daughter Minnie (Tunisha Sharma), who is paralyzed from the waist down and uses a wheelchair. Minnie is a happy child and Vidya dotes on her, working hard to save money so that she can take Minnie to America in the hope that the girl will walk again.
One day, Vidya returns home in the evening to find her daughter is missing. Vidya then receives a call from an unknown man asking her to come to an address if she does not want to see Mini dead. When she crosses the road, a taxi knocks her down, sending her into a coma. The taxi driver takes her to a nursing home run by Dr Maity (Pradip Mukherjee). Police Sub-Inspector Inderjeet Singh (Arjun Rampal) is put on Vidya's case. He visits Vidya's home and finds her diary, which he takes as evidence. Inderjeet's senior, Pranab Halder (Kharaj Mukherjee), informs him that a woman named Durga Rani Singh is wanted for a kidnapping and murder case.
As Inderjeet reads Vidya's diary, her past is revealed. She worked as a clerk in a school in Kalimpong as Durga Rani Singh. She is friendly with Arun (Tota Roychoudhury) and it is obvious that Arun has feelings for her. At school, she noticed that a student named Minnie Dewan (Naisha Khanna) was always punished for sleeping during classes. When after repeated attempts at befriending reclusive Minnie fail, Durga decides to visit Minnie's grandmother (Amba Sanyal), pretending to be a teacher. Durga's worst fears come true when Minnie confides that her uncle Mohit Dewan (Jugal Hansraj) was sexually abusing her. It is now clear from Durga's diary that she was also a survivor of child sex abuse and she informs Minnie's grandmother but she admonishes her. Durga tries to file a case with the police but the policewoman (Gargi Bharadwaj), taking the side of Minnie's grandmother, threatens Durga not to pursue the matter. Minnie jumps off the terrace in desperation and becomes paralyzed from the waist down. Durga decides to save Minnie from her family and has a fight with her grandmother, who gets killed accidentally in the incident. Durga flees with Minnie, settling in Chandan Nagar, with a new name, Vidya Sinha. But Mohit hunts them down and gets Minnie kidnapped with the help of the policewoman.
In the present, the kidnapper policewoman comes to the nursing home and threatens Vidya to pay ransom to save Minnie, who has been kidnapped by Mohit and her. Terrified, Vidya escapes from the hospital after recovering from the coma and returns home in search of the address that the kidnapper asked her to come to. She is caught by Halder and Inderjeet at her home. However, she is successful in escaping from there, as she shoots Inderjeet in his arm. Vidya then arrives at Kolkata, where she goes to the docks as ordered by Mohit. Vidya offers to pay the ransom to the kidnapper policewoman. Mohit shoots the policewoman when she takes interest in the ransom money. He then tries to strangulate Vidya but is shot dead by Minnie, who picked up his fallen revolver. Vidya then calls Inderjeet to seek his help, but Haldar and other police officer ask Inderjeet to call her to a place where they can arrest her. Inderjeet asks Vidya to come to his paternal house, where they are surrounded by the police force. Halder tries to come near to Vidya, but she shoots him in the leg. Later, she sets the house on fire.
It is finally revealed that Vidya's real name is Durga Rani Singh and she was married to Inderjeet Singh at a very young age but got separated later. Durga after recovering from the coma asked Inderjeet for his help and they planned her escape. It was Inderjeet, who helped Durga in arranging the real passports for Vidya and her adopted daughter. He helped her in escaping to Kolkata in the police Ambassador car, inside the car trunk, and he helped her escape from the burning house.
It is seen that Durga is alive, (who is believed to be dead by the police) and is travelling to New York City for Minnie's operation. Inderjeet is seen happily driving with his family.