Summary North Bihar 1952. Dr Ishan Majumdar was found dead in a fort where he lived alone. His body was full of snake poison but had a huge blow on his head & a gold coin in his hand. The fort was owned by Ramkishore (Govind Namdeo), who lived with his family a short distance away from the fort in his mansion. His daughter had also died from snake bite. The mansion is inhabited by Ramkishore, 3 sons (Bansidhar (Narain Pati), Murlidhar (Ajay Srivastava) & a young boy Gadadhar (Master Babloo)), 1 remaining daughter Tulsi (Shilpa Tulaskar) & his son-in-law (Manilal (Rakesh Dube)) husband of the elder daughter who died. A tutor Ramapati (Sashi Hans) comes to teach the younger daughter. Ramapati tells Byomkesh that Ishan was very learned & knew the entire family history of the fort. Ramkishore was Ishan's friend & thought maybe Ishan found his family treasure hidden somewhere in the fort, which led to a gold coin found in his hands. Ramkishore also says that Ishan borrowed a torch a few days before his death, which is now missing.

Ramkishore knows that his sons will bicker over his estate, so he wants to settle matters before he dies. He plans to divide the estate between his 3 sons, but wants to give the fort to Tulsi, whom he plans to get married to Manilal. The 2 elder sons are dead opposed to the same. Byomkesh finds a diary in Ishan's living quarters which talks about a hidden treasure in the fort and indicates that Ramkishore's elder brother Ramvinod knew about the same. But he died 30 years ago.

Byomkesh talks to Tulsi who likes Ramapati & dislikes the fact that Manilal is jealous of him. Tulsi doesn't like Manilal and doesn't want to marry him. A hermit also lives in the jungle surrounding the fort & is supported by alms provided by Ramkishore. Byomkesh investigates the fort further & eventually finds the hidden chamber. He finds the missing torch, but no hidden treasure. The earthen pots in the chamber are empty. Ramkishore and the brothers fire Ramapati, over an alleged theft of a fountain pen from Manilal's room. Tulsi is adamant that she gave one fountain pen to Ramapati & that Manilal had 2, but no-one listens to her. Manilal claims that he only had 1 fountain pen. Byomkesh approaches the hermit who promises to tell him everything at night. But he is found dead when Byomkesh goes to meet him at 10 pm, snake bite marks on his leg.

Byomkesh confronts Ramkishore in front of Manilal & accuses Manilal of murdering the hermit, who was Ramvinod. They use the pretext to search the house, and search Manilal's room first & find the 2 fountain pens that are found to be full of cobra venom. Manilal confesses & Byomkesh starts his debrief. Ramkishore and Ramvinod were ordinary business men 25 years ago. Due to plague Ramkishore ran away & didn't know that Ramvinod survived. Manilal had designs on Ramkishore's wealth from the beginning. Manilal acquired snake venom from vagabonds passing thru a few years ago. His wife, Ramkishore's elder daughter, suspected Manilal & hence she became his first victim.

Manilal acquired Ramkishore's sympathy, who prepares to get Tulsi married to Manilal. That's when Ishan & Ramapati arrive on the scene. Ishan perhaps knew Ramvinod & got him to come to the estate as a hermit & live there. Ishan found the hidden treasure in the fort, but needed help. He chose Manipal, over Ramapati. Deal was to split the wealth 50:50. But Manipal killed Ishan & didn't bother to check that there was no treasure in the hidden chamber. By the time he got ready, Byomkesh arrived on the scene. Things started to unravel when Tulsi gave the pen to Ramapati. He witnessed Byomkesh talking to the hermit & decided to kill him before Sadhu / Ramvinod could tell Byomkesh anything.

Byomkesh knows the real location of the treasure, but makes Ramkishore promise 2 things: he will get Tulsi married to Ramapati and transfer the treasure to them. Then Byomkesh reveals that Rajaram and Jayaram were 2 brother ancestors of Ramkishore. One phrase in Ishan's diary said that if Rajaram and Jayaram are dead, then gold be safeguarded by Mohanlal. Mohanlal is the name of a canon in the fort. Rajaram had liquefied the gold & stuffed it into a single column in the barrel of the cannon named Mohanlal.

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