It's 1942 and Calcutta is ruled by the British Raj. Its underworld is ruled by Chinese gangsters smuggling opium from Shanghai. A vicious fight breaks out within one such gang at the Calcutta docks where gangster Yun Gong brutally blinds one rival and is then thought to be killed.
Meanwhile, bright student Byomkesh Bakshy (Sushant) is approached by Ajit Banerjee for help in finding his missing father Bhuvan babu. Initially, Byomkesh refuses and even suggests Bhuvan may be mixed up in dodgy business, at which Ajit angrily slaps him. The same day, Byomkesh's girlfriend Leela tells him she's marrying a well-placed chemistry student instead of him.
Byomkesh seeks out Ajit and agrees to take his case. He visits Bhuvan's lodging home and meets Dr. Guha and Kanai Dao who legally sells opium. He also finds Bhuvan's beloved paan box is still in his room, hidden by his roommate who's oddly addicted to the paan masala. Byomkesh is convinced Bhuvan has been killed as his box and money are still in his room.
Byomkesh visits the Sikdar chemical factory, now shut down, where Bhuvan used to work. There he sees film star Angoori taking a swim. She's amused by him and offers him a ride home. In the car, when he asks if she knows of Bhuvan, she pulls her handbag nervously to herself. In her studio, as she shoots, Byomkesh opens the bag and finds letters from cut-out newspapers, blackmailing the factory owner Sikdar babu, a friend of Angoori's.
He's convinced Sikdar has killed Bhuvan when the latter tried to blackmail him. Byomkesh goes back to the factory with Dr. Guha and they pretend to the watchmen that they've come to fix a mouse problem. Poking around in the factory, they find Bhuvan's slashed, decomposed body stuffed into a machine.
Sikdar is brought in by police commissioner Wilkie for questioning - but just then, Byomkesh discovers he's wrong and Sikdar is being framed as the newspapers only reach the lodging house at 8:30 AM every day while the blackmail letters were stamped at 7:30 AM at the post office, showing a false detail. Dr. Guha vanishes but sends Byomkesh a note, admitting he was blackmailing Sikdar, not Bhuvan babu.
Sikdar is freed and Byomkesh follows him home but is detained by Angoori who tries to kiss him. He gently rebuffs her but when he goes to meet Sikdar, the latter is suddenly poisoned and dies. Suspicion falls on his rebellious nephew Sukumar who has opened a rival political party and is militant about India's freedom from the British. Sukumar's sister Satyawati pleads with Byomkesh to help her brother, who's driven off earlier in a cab used by Byomkesh.
Byomkesh finds the Sardarji cabbie Sukumar drove off with, reaching a Japanese dentist Dr. Watnabe's clinic where the secretary says Watanabe is at the temple. Byomkesh and Ajit go to the temple, pretending they have a link with Sukumar's political party, but Watanabe is suspicious. Another member of Sukumar's party recognizes them and follows them at Watanabe's orders. Byomkesh and Ajit change their costumes and then follow the student back to the dentist's - where they find the secretary and student brutally murdered with a shadowy character escaping in a car.
Byomkesh takes a paper out from the student's pocket - it's a map of Calcutta's river route. They return to the lodgers' house where Byomkesh feels low. He tries some of Bhuvan babu's paan masala and finds it is laced with heroin. Making a wall-painting of the entire crime, Byomkesh has the heroin presence confirmed by Leela's chemist husband who says this heroin is very hard to trace and made from opium - which Dao, who admits he's an undercover cop, tells Byomkesh has practically vanished from the city.
Byomkesh finds there are two plots together here. The first is the Chinese gang warfare over opium, turned into untraceable heroin by Bhuvan babu's secret formula. The second is of Sukumar's political party being told by the Japanese army that they will land in Calcutta via its river routes and liberate the city from the British.The link between the two deadly plots is the mysterious Yun Gong.
Meanwhile, Dr. Guha returns to the lodging house and tells Byomkesh he's a nationalist fighting for India's freedom. He offers Byomkesh a role to play in the struggle and says other peoples' murders are the price for this. Byomkesh asks Dao to take him to the Chinese gang which tells him about the deadly Gong. Byomkesh divulges all the facts to commissioner Wilkie and asks him to sound all the air raid sirens of Calcutta at 4 AM on Basant Panchami. He is convinced the Japanese will attack Calcutta through its river routes then and Gong will help them so that they let him control the hugely rich opium trade.
Byomkesh sends Dr. Guha a letter inviting him to the lodging house. Dr. Guha, Sukumar, Watanabe, and Satyawati come there, as does Angoori who's told Byomkesh she's loved Gong from their days together in Rangoon and has been trying to mislead Byomkesh on Gong's orders. She hasn't seen him recently though and doesn't recognize him - not even when Byomkesh reveals Dr. Guha is actually the dreaded Gong.
Angoori tries to stop Gong from helping the Japanese conquer Calcutta but he brutally murders her. The Chinese gang hid by Byomkesh in the lodging house then attacks and takes away Gong who gets distracted by the air sirens going off. Byomkesh tells the Japanese doctor their game is up and the British army will crush their forces if they try to attack. He gives up.
Byomkesh then tells Satywati he wants to marry her as the price for proving Sukumar's innocence. Meanwhile, Gong manages to kill the entire Chinese gang and escapes - promising Byomkesh revenge.