In 1942, amidst British-ruled India, chaos erupts at the Calcutta docks when a violent clash occurs between rival gangs. The notorious gangster Yang Guang brutally blinds a competitor and is assumed dead in the melee.
Some time later, a student named Byomkesh Bakshy is approached by his classmate Ajit, whose father, Bhuvan Banerjee, a chemist, has inexplicably vanished for over two months. Initially, Byomkesh is reluctant to assist, even suggesting that perhaps Bhuvan has engaged in dubious activities, which angers Ajit enough to slap him in confrontation. Meanwhile, Byomkesh’s girlfriend, Leela, has plans to marry another chemistry student, further complicating his emotions.
Eventually, intrigued by Ajit, Byomkesh agrees to investigate. Examining Bhuvan's belongings, he senses that there’s more to this case than meets the eye. His search leads him to Bhuvan's lodging, where he questions residents such as Dr. Guha, Ashwini Babu, and Kanai Dao, the last of whom is known as a licensed opium merchant. A significant clue appears when Byomkesh discovers Bhuvan's hidden Paan box, indicating he had no intention of leaving, thus inferring his likely death.
In a pivotal encounter at the factory where Bhuvan worked, Byomkesh meets actress Angoori Devi, who nervously clutches blackmail letters directed at Gajanan Sikdar, a local politician who owns the factory. Guided by Dr. Guha, Byomkesh deduces that Bhuvan attempted to blackmail Sikdar, leading to his murder. Their investigation takes a dark turn when Bhuvan's decomposed body is discovered, concealed within the factory machinery.
As they pursue the case, police commissioner Wilkie interrogates Sikdar. Just when Byomkesh believes the clues are leading to a resolution, he finds himself detained at the station for impersonating an officer. Following his release, he trails Sikdar home but is intercepted by Angoori, who attempts to seduce him. Their encounter is cut short when Sikdar is found poisoned shortly after, and suspicion shifts to Sukumar, his rebellious nephew.
Seeking justice, Byomkesh is drawn deeper into a web of conspiracy, discovering the involvement of Dr. Watanabe, a Japanese dentist, and the looming threat of the Japanese army's plans to attack Calcutta. A clue—a map of Calcutta’s river routes—leads Byomkesh to serious revelations about Bhuvan's secret formula for opium, which has fallen into dangerous hands.
At a crucial confrontation, Dr. Guha appears, revealing his nationalistic intentions and claims that the unfolding murders were merely collateral damage in a larger fight for India’s freedom. Byomkesh refuses his offer to join, aware that the stakes are higher than personal greed; they involve the very fate of Calcutta.
In a nerve-wracking finale, Byomkesh notifies the police of imminent danger, accurately predicting when the Japanese will strike. The uneasy alliance with the Chinese gang, who are also on Guang's side, proves pivotal in the ensuing chaos. In a dramatic turn, Angoori confronts Guang, but her attempt to sway him leads to her brutal demise.
As the air raid sirens sound, Byomkesh concocts a ruse to thwart the impending attack, ultimately confronting both Guang and the Japanese army. In an explosive determination to prevent catastrophic results, Byomkesh fights against a tide of darkness, realizing that the true cost of honor and allegiance might mean sacrificing his desires, culminating in a heartfelt proposal to Satyawati.
In a chilling epilogue, it is revealed that despite the chaos, Yang Guang survives, having extinguished his rivals and vowing revenge against Byomkesh, setting the stage for a perpetual game of wits amidst the shadows of Calcutta.
