Kahaani

A poison-gas attack cripples a Kolkata Metro Rail compartment, killing over 100 passengers. Arup Basu boards the train hoping to recover a bag he suspects carries the toxin, only to realize the danger lies with a milk bottle belonging to a frightened mother. The attack triggers a high-profile but ultimately inconclusive investigation.

Investigators struggle to find a lead as the case lands at Kalighat Police Station. Despite multiple inquiries, the trail goes cold and public memory of the incident fades. The case becomes a forgotten scandal.

Two years after the Metro attack, Vidya Bagchi arrives in Kolkata from London during Durga Puja in search of her missing husband Arnab. She is pregnant and claims to be a software engineer, determined to uncover Arnab's fate. The city greets her with festival energy while she pursues a personal mission.

SI Chatterjee and the helpful policeman Rana take Vidya's case seriously, offering to help in the search for Arnab. Vidya asserts Arnab worked for the National Data Center and was there on a two-week assignment. Their investigation outlines a missing person whose existence is hard to verify.

Vidya checks into room 15 at the Mona Lisa guest house, the last known place Arnab was staying according to rumors. The lack of records deepens the mystery as inquiries fail to locate Arnab or confirm his assignment. The hunt widens beyond official channels.

Rana takes Vidya to Arnab’s supposed hometown, where locals remember no Bagchi family and the local school has no Arnab on its rolls. There is no record of Arnab flying from London or landing in Kolkata. The trail suggests Arnab may never have existed as described.

Agnes D'Mellow, the NDC's head of HR, hints that Arnab resembled a rogue IB agent. She is killed by Bob Biswas, who is undercover as a life-insurance agent, at the entrance of her house. The murder halts further assistance and confirms shadowy stakes around Arnab’s identity.

Vidya and Rana break into the NDC office and discover Milan Damji's file, narrowly escaping Bob who is also after the same records. The pursuit catches the attention of IB officials Khan and Bhaskaran in Delhi. The Damji file links Arnab to a larger conspiracy.

IB officials arrive in Kolkata; Khan reveals that Damji was a rogue IB agent connected to the Metro attack, while Bhaskaran warns Vidya off. Despite the danger, Vidya presses on with her search for the truth.

Damji’s rare blood group is traced to a blood bank and a nursing home, implying a secretive link to the Metro attack. The nurse who treated Damji is connected, and the doctor is later killed by Bob. These threads pull Vidya deeper into the conspiracy.

An informer describes a clandestine meeting between Damji, Vijay Verma, and Dinesh; Damji interrupts and kills Vijay and Dinesh, wounded in the scuffle. Vidya suspects someone high up in the IB HQ tipped Damji off. The investigation points toward an internal conspiracy.

A data trail from Sridhar's computer deciphers a code that reveals Bhaskaran's phone number. Vidya uses it to contact Bhaskaran, exchanging access to Damji's records for information about Arnab. An unknown caller later warns her to surrender the documents.

Vidya and Rana confront Damji, and a violent confrontation ensues as Vidya uses a prosthetic pregnancy belly to disarm him and shoots him with his own gun. She escapes into the crowd, leaving a pen drive with data that leads toward Bhaskaran's arrest. Khan and Bhaskaran close in on the trail.

The pen drive leads to Bhaskaran’s arrest, and Rana concludes that Vidya and Arnab never existed, exposing Vidya’s use of the police and IB for her own ends. It is revealed that Vidya is the widow of Arup Basu, an IB officer killed in the Metro attack, and she was aided by retired IB officer Pratap Bajpayee. The story hints at a top IB official as the mastermind behind it all.