A poison-gas attack on a Kolkata Metro Rail compartment kills the passengers on board. Two years later Vidya Bagchi (Vidya Balan), a pregnant software engineer, arrives in Kolkata from London during the Durga Puja festivities in search of her missing husband, Arnab Bagchi. A police officer, Satyoki "Rana" Sinha (Parambrata Chatterjee), offers to help. Although Vidya claims that Arnab went to Kolkata on an assignment for the National Data Center (NDC), initial investigations suggest that no such person was employed by the NDC.

Agnes D'Mello, the NDC's head of human resources, suggests to Vidya that her husband resembled former employee Milan Damji (Indraneil Sengupta), whose file is probably kept in the old NDC office. Before Agnes can provide any further help she is killed by Bob Biswas (Saswata Chatterjee), an assassin working undercover as a life insurance agent. Vidya and Rana break into the NDC office and find Damji's file, barely escaping an encounter with Bob, who is searching for the same information. Meanwhile, the attempts to obtain Damji's records have attracted the attention of two Intelligence Bureau (IB) officials in Delhi—the chief Bhaskaran K. (Dhritiman Chatterjee) and his deputy Khan (Nawazuddin Siddiqui). Khan arrives in Kolkata and reveals that Damji was a rogue IB agent responsible for the poison-gas attack. In spite of Khan's warnings, Vidya continues her search, fearing that Arnab's resemblance to Damji may have led him into trouble.

The address on Damji's record leads Vidya and Rana to a dilapidated flat. An errand boy from the neighbourhood tea stall identifies R. Sridhar (Shantilal Mukherjee), an NDC officer, as a frequent visitor to Damji's flat. Bob attempts to kill Vidya, but fails, and is run over by a car during a chase. Examination of Bob's mobile phone leads Vidya and Rana to an IP address sending instructions to kill her. They break into Sridhar's office to verify his IP address, but he is alerted electronically and returns to his office. Vidya accidentally shoots Sridhar dead during a scuffle, which upsets Khan, who had wanted him alive.

Sridhar's computer data reveals a code, which when deciphered reveals Bhaskaran's phone number. Vidya calls Bhaskaran to tell him that she has retrieved sensitive documents from Sridhar's office. She asks Bhaskaran to help find her husband in exchange for the documents, but Bhaskaran tells her to contact the local police. Vidya soon gets a call from an unknown number however, warning her that she should hand over the documents to the caller if she wishes to see her husband alive. Khan thinks the caller is Milan Damji.

Vidya goes to meet Damji, followed by Rana and Khan. Damji cuts the meeting short when Vidya expresses her doubt that he will be able to return her husband in exchange for the sensitive file, and he attempts to leave. Vidya tries to stop him, and in the ensuing struggle Damji draws a gun on her. Vidya disarms him using the prosthetic belly she has been using to fake her pregnancy and promptly stabbing him in the neck with her hair stick, after shooting him with his own gun. She flees into the crowd before the police arrive, leaving a thank you note for Rana and a pen drive containing data from Sridhar's computer, which leads to Bhaskaran's arrest. Rana concludes that no Vidya or Arnab Bagchi ever existed, and that Vidya had been using the police and the IB to achieve her own ends.

Vidya is revealed to be the widow of Arup Basu (Abir Chatterjee), an IB officer and Damji's colleague, who was killed in the poison-gas attack, which also caused Vidya to immediately fall unconscious upon seeing her husband's corpse and suffer a miscarriage. In her mission to avenge his and their unborn child's death, Vidya was helped by retired IB officer Pratap Bajpayee (Darshan Jariwala), who suspected the involvement of a top IB official.

• Vidya Balan as Vidya Venkatesan Bagchi
• Parambrata Chatterjee as Satyoki "Rana" Sinha
• Nawazuddin Siddiqui as A. Khan
• Indraneil Sengupta as Milan Damji
• Dhritiman Chatterjee as Bhaskaran K.
• Saswata Chatterjee as Bob Biswas
• Darshan Jariwala as Colonel Pratap Bajpayee
• Abir Chatterjee as Arup Basu
• Shantilal Mukherjee as R. Shridhar, chief technical officer at the NDC
• Kharaj Mukherjee as Inspector Chatterjee, a friendly inspector at the Kalighat Police Station.
• Colleen Blanche as Agnes D'Mello
• Nitya Ganguli as Mr. Das, the guest house owner.
• Ritabrata Mukherjee as Bishnu, a worker at the guest house.
• Pamela Bhuttoria as Sapna, an employee at NDC.
• Kalyan Chatterjee as Paresh Pal, a clay artist and a police informer.
• Riddhi Sen as Poltu, a worker at a tea-stall.
• Massod Akhtar as Rasik Tyagi, systems supervisor at the NDC

Development

Sujoy Ghosh approached novelist and script writer Advaita Kala with the idea for the film. Kala took inspiration from her experience in Kolkata, where she had moved in 1999, akin to the protagonist in the film. She reported that despite facing a language barrier and the chaos and poverty of the metropolis, she was charmed by the warmth of the people, which was reflected in the film. Kala started writing in 2009 and finished the 185-page script by February 2010. Her research included reading the books Open Secrets: India's Intelligence Unveiled by Maloy Krishna Dhar and India's External Intelligence: Secrets of Research and Analysis Wing (RAW) by V. K. Singh.

Ghosh, who co-wrote the story and the screenplay, began to plan the film while awaiting the release of his previous film Aladin (2009), but the dismal response to Aladin was a setback. He had to approach several producers to finance him for Kahaani, but was refused and discouraged from making the film owing to three factors: a pregnant woman as the lead star, a bunch of unknown Bengali actors as the supporting cast and Kolkata as a backdrop. Yashraj Films were willing to produce the film, but wanted Ghosh to sign a three-film deal, which he declined because he did not want that much commitment.

Bengali film actor Prosenjit Chatterjee encouraged Ghosh to shoot in Kolkata. Ghosh finally selected Kolkata for several reasons: the director's acquaintance with the city, its mix of modernity and old-world charm, and budget constraints. Kolkata is a cheaper location than Mumbai or Delhi, where most Bollywood films are shot.

Ghosh admitted in an interview that after his two preceding directorial ventures—Aladin and Home Delivery (2005)—performed poorly at the box office, Kahaani was his last chance to create a niche as a director. He added that the film's plot twist came somewhat accidentally. Having described the skeleton of the story to a friend during its development, the friend called him back a few days later to enquire about his film. The friend had mistakenly imagined sequences which he assumed to be parts of the plot, from which the twist ending was derived.

Casting

Vidya Balan was the first choice of both writer Advaita Kala and director Sujoy Ghosh. Ghosh had planned for a long time to work with Vidya and approached her in 2010. Unimpressed with the plot outline, Vidya refused, only changing her mind after having read the completed script.

Ghosh chose mostly Bengali actors as he wanted to make the characters as authentic as possible. The role of the Inspector Satyoki "Rana" Sinha was first offered to Chandan Roy Sanyal, but he could not take the part due to other commitments. Parambrata Chatterjee, a Bengali actor whose acting in the film The Bong Connection (2006) had impressed Ghosh at the Mumbai Academy of the Moving Image festival, was later offered the role in Kahaani. Chatterjee had earlier worked with Vidya in her début film Bhalo Theko (2003).

The casting director Roshmi Banerjee suggested Nawazuddin Siddiqui for the role of Khan. Siddiqui, who had only had minor roles in Bollywood up to that time, was surprised that for the first time he would not have to portray a beggar. Saswata Chatterjee, another Bengali actor, was surprised as well when he was offered the role of the contract killer Bob Biswas. He thought there were suitable actors in Hindi film industry for the role. He said that Ghosh had known him since childhood and was impressed with his acting, so he wanted him as Bob Biswas.

Ghosh went against the expectations of casting a popular actor from Bollywood. He signed Bengali actor Abir Chatterjee to play Vidya's husband. According to Ghosh, popular Bollywood actors were not willing to work with him after his two previous flops. He also believed that audience might expect more screen-time from a better-known actor. Several other Bengali film and television actors, such as Indraneil Sengupta and Kharaj Mukherjee, were cast in supporting roles.

Characters

Before the shooting of the film began, Vidya started to use a prosthetic belly to look as close to authentically pregnant as possible. According to news reports, she met doctors and pregnant women to learn about the typical lifestyle and nuances of a pregnant woman, and also made lists of rules and superstitions followed by pregnant women. Vidya said that during her college days she often used to imitate pregnant ladies during stand-up acti