A poison-gas attack on a Kolkata Metro Rail compartment ignites a harsh, sprawling mystery that cuts through a city and leaves more questions than answers. The tragedy claims the life of Arup Basu, who was on the train that day and had boarded in search of a bag believed to hold the toxin. He suspects the bag is with a frightened schoolchild, but the deadly cargo is actually hidden inside a milk bottle carried by a mother with an infant. When the bottle shatters on the train floor, the gas is released and more than a hundred passengers lose their lives in an instant.
The initial investigation stalls and the case seems to drift into silence, slipping under the radar of the Kalighat police station that takes up the case. Two years pass, and Vidya Bagchi Vidya Bagchi, a pregnant software engineer who has traveled from London to Kolkata, arrives during the Durga Puja festival in search of her missing husband. She states that Arnab Bagchi Arnab Bagchi, an IB-friendly tech expert, had come to the city for a two-week assignment with the National Data Center (NDC). Vidya emphasizes that Arnab vanished without a trace after those two weeks, and she insists on finding him despite a lack of official records.
At Kalighat, Sub-Inspector Chatterjee Sub-Inspector Chatterjee offers a cautious line of inquiry, and Vidya reluctantly gains an ally in Asst Sub-Inspector Satyoki "Rana" Sinha Satyoki "Rana" Sinha, who agrees to help navigate a maze of dead ends. Vidya presents a story of Arnab’s supposed assignment with the NDC, yet the official files suggest no such employee exists. The Mona Lisa guest house, where Arnab was allegedly staying, has no record of his presence either. Vidya moves into room #15, the very room linked to Arnab, and Rana begins tracing the lifelong footprint of the Bagchi family in the city.
As the search advances, Agnes D'Mello Agnes D'Mello, the NDC’s head of human resources, hints that Arnab may have resembled a former employee, Milan Damji, whose records are tucked away in the old NDC files. Before Agnes can offer more help, she is fatally shot at her doorstep by Bob Biswas Bob Biswas, an undercover assassin posing as a life-insurance agent. Vidya and Rana break into the NDC offices, unearthing Damji’s file and narrowly escaping Bob’s pursuit, while two intelligence officers in Delhi—Bhaskaran K. Bhaskaran K. and Khan Mr. Khan—watch the events closely from afar.
Khan arrives in Kolkata and goes public with a startling reveal: Damji was a rogue IB agent implicated in the poison-gas attack. Despite the warning, Vidya persists, fearing Arnab’s potential danger if his resemblance to Damji is true. Bhaskaran reaches out to retired Colonel Pratap Bajpayee Pratap Bajpayee to discuss Damji, setting the stage for deeper involvement from the old-guard IB network. Vidya and Rana dig further, discovering Damji had a rare blood group and had stored blood in a local bank for a nursing home on the night of the attack. They locate the treating doctor, only to have that doctor shot by Bob when he is pulled away from a surgery.
An informer reveals a meeting the night of the attack between three people—Damji, Vijay Verma, and Dinesh—who were exchanging information before a deadly collision of interests. Rana suspects this meeting hints at Damji’s plan being orchestrated by someone high up in the IB hierarchy. The search leads Vidya and Rana to a dilapidated flat, where an errand boy from a neighborhood tea stall identifies R. Shridhar R. Shridhar as a frequent visitor to Damji’s place. A tense chase ensues: Bob tries to kill Vidya but is run over during the pursuit, and Vidya’s discovery of an IP-address-linked target sends them to Sridhar’s office. When Sridhar is alerted and returns, Vidya, in the heat of the moment, shoots him dead.
Sridhar’s computer holds a code that, when deciphered, reveals Bhaskaran’s phone number. Vidya uses this to broker a deal with Bhaskaran, offering the damning documents in exchange for help finding Arnab, but Bhaskaran deflects and tells her to involve the local police. A mysterious caller then warns Vidya to hand over the materials if she hopes to see her husband alive. Khan suspects Milan Damji is behind the call.
Vidya agrees to a meeting with Damji, followed by Rana and Khan. Damji abruptly ends the encounter and attempts to flee; Vidya fights back and disarms him using a prosthetic belly she has been employing to feign pregnancy, then shoots him with his own gun. She escapes into the crowd, leaving behind a folded note for Rana and a pen drive containing Sridhar’s data, which leads to Bhaskaran’s arrest. Rana concludes that Vidya and Arnab never existed in the way they believed, suggesting Vidya has been manipulating the police and the IB to advance her own objectives.
In the final twist, Vidya is revealed to be the widow of Arup Basu—the IB officer who died in the poison-gas attack. Her vendetta to avenge his death unfolds with the help of retired IB officer Pratap Bajpayee, who had long suspected the involvement of a high-ranking IB official. The story closes on a note of moral ambiguity, presenting Vidya as both a grieving widow and a calculating force—someone who weaponizes the truth to uncover a conspiracy that reaches into the very heart of the city’s security apparatus. The film leaves viewers with a stark question about justice, memory, and the price of unveiling the truth in a web of shadows.
