• Composer : Sneha Khanwalkar
• Lyricist : Dibakar Banerjee , Sneha Khanwalkar
• Singer : IJA

The film begins in November 1942 in Calcutta, where a group of Chinese men from the Green Gang are smuggling opium from the docks. An unknown gang attacks them, and one of them brutally blinds the Chinese gang's leader. In January 1943, Ajit Kumar Banerjee (Anand Tiwari) approaches Byomkesh Bakshi (Sushant Singh Rajput) asking him to locate his father, Bhuvan Banerjee, who has been missing for two months. Byomkesh rejects the request initially but later takes on the case. The following day, he visits the lodging house Bhuvan Babu resided in, run by Dr. Anukul Guha (Neeraj Kabi), a physician. Byomkesh also meets Kanai Dao (Meiyang Chang), an opium seller and Ashwini Babu, Bhuvan Banerjee's roommate. Byomkesh finds Bhuvan Babu's paan box, containing his custom-made formula, which was stolen by Ashwini Babu, who was addicted to the formula. Byomkesh then deduces that Bhuvan Banerjee has been murdered. He visits the closed chemical factory where Bhuvan Babu used to work, where he meets an actress, Anguri Devi (Swastika Mukherjee), the wife of the factory's owner, Gajanan Sikdaar (Dr. Kaushik Ghosh). While Byomkesh interrogates her, he notices her nervously hiding her purse. When Anguri goes to film a scene, Byomkesh goes through the contents of the purse and finds a set of letters containing words cut out from the newspaper, blackmailing her husband, Gajanan Sikdaar. Byomkesh deduces that Gajanan Sikdaar has killed Bhuvan Babu, who was blackmailing the former. Byomkesh visits Gajanan Sikdaar's residence, where Sikdaar's nephew Sukumar is creating his own political party against Sikdaar. Byomkesh is thrown out of the house when he tries to ask Gajanan Sikdaar about Bhuvan Babu. Byomkesh and Dr. Guha go back to the factory and sneak in after conning the watchmen. While inspecting the factory, they find Bhuvan Babu's slashed, decomposed body stuffed into a machine. Sikdaar is brought in by Deputy Commissioner Wilkie for questioning. However, Byomkesh realizes from the dates on Bhuvan Babu's letters that they are fake, and Sikdaar is being framed. Sikdaar is freed, and Byomkesh follows him home but is stopped by Anguri Devi, who attempts to seduce him. He gently rebuffs her and goes to meet Sikdaar, who immediately has a fit and passes away, having been poisoned. Suspicion falls on Sukumar, who was seen making death threats to Sikdaar. Dr. Guha leaves Calcutta temporarily but sends Byomkesh a letter and the stamp used on the fake letters, admitting that he was the one who faked the letters and killed Bhuvan Babu. Satyavati (Divya Menon), Sukumar's sister, pleads with Byomkesh to help her brother, who she believes is innocent. Byomkesh finds the cab Sukumar was last seen in, and the driver drops him at a Japanese dentist, Dr. Watanabe's office, from where Sukumar was seen driving off with Watanabe. Byomkesh and Ajit go to meet Watanabe, saying they are part of Sukumar's Spring Day campaign, but Watanabe realizes they are imposters. Watanabe sends another member of Sukumar's party to follow the duo. Byomkesh and Ajit, realizing they are being followed, change their attires and then follow the young man back to Watanabe's office. Realizing the duo have outsmarted him, Watanabe murders the follower and his receptionist and escapes from there. Byomkesh takes a paper out from the follower's corpse's pocket, which contains a drawing Byomkesh realizes he has seen before. Meanwhile, Dr. Guha returns to the lodging house and meets Byomkesh and Ajit, admitting that he committed the murders as he and Sukumar are working in cahoots to fight for the country's freedom, saying Bhuvan Babu and Sikdaar were killed for being traitors. Upon returning to their room, Ajit begins venting about his father and the latter's overcritical treatment of Ajit, during which he reveals Bhuvan Babu had lost a job at Hind Chemicals when they realized that his master formula had contained opium, used to addict customers. Byomkesh then tries Bhuvan Babu's Paan masala, which Ashwini Babu appears to be addicted to, and in a state of euphoria, creates a wall-painting of his understanding of the entire case, saying Sikdaar had said the words “Young Gun” in Byomkesh's ear just before dying. Byomkesh has the paan and his own blood tested; while his blood has traces of heroin, the paan is clear, and he deduces that the heroin was made by Bhuvan Babu with the property of not being traceable until it has been consumed. Byomkesh learns from Kanai Dao that the opium supply in the local areas has been halted, and asks him if he can help Byomkesh meet any heroin suppliers. However, Kanai turns out to be a police informer and turns Byomkesh and Ajit into Deputy Commissioner Wilkie. Wilkie reveals to Byomkesh about the Green Gang and their enmity with one of their former members Yang Guang, who wanted Calcutta's opium business for himself and was killed by the Green Gang but has recently been touted to be alive. Byomkesh realizes Sikdaar had said Yang Guang, not

• Track 1 : Calcutta Kiss ( कलकत्ता चुंबन )
• Track 2 : Bach Ke Bakshy ( बच के बख्श्यो )
• Track 3 : Byomkesh in Love ( ब्योमकेश इन लव )
• Track 4 : Jaanam ( जानेमी )
• Track 5 : Chase in Chinatown ( चाइनाटाउन में पीछा )
• Track 6 : Life's A Bitch ( जीवन एक कुतिया है )