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. However, 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. Vidya also maintains a personal diary where she records their daily lives.
One day, Vidya leaves Mini home alone, as she awakes late. When she reaches work, she confirms that the nurse has arrived to look after Mini. When she returns home, her daughter is missing. A distraught Vidya finds out that the nurse did not go because she received a phone call from someone (pretending to be Vidya), telling her not to come. Vidya then receives a call asking her to come to an address if she does not want to see Mini dead. When she attempts to cross the road, a taxi knocks her down, sending her into a coma. She is admitted by the taxi-driver into a local dispensary of Dr. Maity. A sub-inspector, Inderjeet Singh (Arjun Rampal) is put on Vidya's case. He goes to Vidya's home in search of some evidence & ends up finding her personal diary. He gets a rude shock when he sees Vidya, as her face resembles that of Durga Rani Singh, whom he had known. Inderjeet's senior, Haldar (Kharaj Mukherjee), informs him that Durga is wanted for a kidnapping and murder case.
Vidya's past is shown (when Inderjeet starts to read her diary). She was working as a clerk in a school in Kalimpong as Durga Rani Singh, where she noticed that a student named Minnie Dewan (Naisha Khanna) was always punished for sleeping during class. Minnie hailed from a royal family. Curious to know the reason behind her behavior, Durga visited Minnie's grandmother (Amba Sanyal), pretending to be a teacher. She offered private tuition for her, and won Minnie's heart. Durga then learned that her uncle Mohit Dewan (Jugal Hansraj) is sexually abusing her. When Durga realized that Mohit does this deliberately, she tried to file a case against him. Minnie jumped off the terrace in desperation, becoming paralyzed. Durga decided to save Minnie from her family, and had a fight with her grandmother, who is then killed in the incident. Durga fled with Minnie, settling in Chandan Nagar, with a new name, Vidya Sinha. But Mohit hunted them down and kidnapped Minnie.
The story then comes back to the present time. Vidya escapes from hospital after recovering from coma, & goes back to home in search of the address, that kidnapper had asked her to come at. She is caught by Halder & Inderjeet at her home. However, she is successful in escaping from there, as she shoots Inderjeet in his right shoulder. Vidya then arrive at Kolkata, where she goes to docks area as asked by Mohit (kidnapper). Here Mohit tries to kill Vidya, but he is shot dead by Minnie. 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, as she is a criminal. Inderjeet asks Vidya to come to his paternal house, where they are surrounded by the police forces. Halder tries to come near to Vidya, but she shoots him on his leg. Later, she pours kerosene oil and leaks the gas from cylinder in order to blast the house (in order to kill everybody inside the house). But when, Haldar asks for mercy from her on the behalf of his 8 year-old child, she allows him & Inderjeet to walk out of the house. Few minutes after, house blasts off. In the next scene, It is seen that Haldar comes out of the police headquarters as he has got promotion while Inderjeet is not promoted. Inderjeet becomes sad.
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. Durga after recovering from coma asked Inderjeet for his help & they planned her escape. It was Inderjeet, who helped Durga in arranging fake passports (in the name of Vidya Sinha) for her & for her daughter. He helped her in escaping to Kolkata in the police Ambassador car, under car trunk. Finally, he helped her from escaping from the house which blasted.
It is seen that Durga is alive, (who is believed to be dead as a criminal by police department) traveling to New York for Minnie's operation. They arrive at the Children's Hospital. The film ends with Durga telling Minnie - ?Until I am alive, nothing can happen to youâ€™.