In 1990, the last surviving member of a Murshidabad family of landlords auctions off his valuable belongings, including an ancient sword, for a pittance to the local villagers. That sword is purchased for a mere 1000 rupees by Somnath Das. Soon after, a sharp-eyed businessman, Amirchand Mittal, approaches with a tempting offer: 10 lakh rupees in exchange for the same sword. Somnath declines, setting off a dangerous chain of events. Amirchand is followed by a police officer, who is killed by the businessman’s goons. Somnath becomes the unwitting eye-witness to the murder, and when the goons shoot at him, he miraculously escapes—only to be crushed by a car on the highway moments later.
Fast forward to the present, and Soham and Brishti are touring Kolkata, with Soham narrating the city’s layered history. He hopes to pursue a historical study of Bengal and Kolkata, and Brishti suggests his scholarship be connected to her father, the renowned historian Ashutosh Singha, author of Banglar Ayna. Ashutosh is being interviewed by Jahnabi, an independent documentary filmmaker, when he starts receiving a cascade of unnamed chits that describe how conspirators against Siraj ud-Daulah met their end. Brishti dismisses the notes as pranks, but Soham senses a link to his father’s work and to Somnath’s past. Ashutosh invites Soham to stay at his house, and Soham is introduced to the caretaker, a man named Sakkhi Gopal, who is a distant relative of Ashutosh.
Ashutosh recounts the story of his friend Somnath, who died in a road accident 27 years earlier. Before his death, Somnath hinted at a gift that carried unprecedented historical value and sent a letter to Ashutosh with a clue. A second letter mentions Dinabandhu Mitra’s Neel Darpan and its English translation by Michael Madhusudan Dutt, along with the arrest of Reverend James Long for publishing the play. Meanwhile, Brishti’s brother Abhiraj, pressured by Amirchand over debts, steals a nose-ring sacred to the family deity, drawing Amirchand’s fury over the fake he receives in return.
Soham and Brishti’s search leads them to James Long Sarani, where a shopkeeper from Dinabandhu Mistanna Bhandar performs a peculiar act of burying a ledikeni sweet in the ground. The shopkeeper reveals that his father, the former owner, knew Somnath well and was instructed to perform this act in front of the person who solves the second riddle. Soham deduces that burying the ledikeni symbolizes Lady Canning’s grave in Barrackpore, the sweet made to celebrate her arrival in India. They travel there and encounter a gardener who once knew Somnath; the man is about to reveal their kinship when he is shot dead. Brishti and Soham escape with another letter.
A flash of revelation comes when Brishti spots Jahnabi again. The third clue leads them to a North Kolkata lock-maker’s shop called Black Pagoda, hinted at in the gardener’s letter. The lock has a deadly design: a combination system that detonates gunpowder if a wrong number is entered. After a tense moment conveyed via a telephone call from the father’s shop, the father himself sends another missive, but he is shot before he can reveal more. The nose-ring theft continues to haunt the family, and Amirchand grows more furious with the escalating riddle hunt.
A pivotal moment arrives when Sakkhi Gopal’s remark pushes Soham to confront the truth about Somnath. Soham reveals he is Somnath’s son, a fact that unsettles Brishti and Abhiraj alike. Ashutosh’s betrayal becomes clear: the chits were orchestrated to pressure him into selling the gift, and Sakkhi’s confession also points to Soham’s true parentage.
Brishti and Soham press on to Murshidabad, guided by the final hints. The last riddle leads them to Khoshbagh, Siraj ud-Daulah’s final resting place, and to the Katra mosque, where the gift is hidden. When Amirchand and his goons arrive, they start digging, ignoring Soham’s warning not to touch the prize. The treasure is revealed to be the very sword Siraj ud-Daulah wielded, a relic of immense historical value gifted by French traders. At the moment Amirchand touches it, an electric shock from a high-voltage wire—laid to safeguard the artifact—knocks him out. Jahnabi steps in as a CID officer investigating the 1990 murder, disclosing herself as the daughter of the murdered officer, and she rescues Soham and Brishti.
In the aftermath, Ashutosh is arrested as an accomplice, and the sword is entrusted to the museum for safekeeping. He confesses that he published Somnath’s Banglar Ayna under his own name, tying the chits to his betrayal. The film closes with a quiet, metaphorical glimpse of Soham’s father, leaving the audience with a sense of unfinished debts and legacies left to be understood.
