Byomkesh, a detective, stumbles upon a case of robbery while recuperating from a prolonged illness. With the only object to be stolen being a group photograph, he attempts to solve the mystery.

Based on the novel Magno Mainak by Sharadindu Bandyopadhyay, the film portrays the iconic Bengali detective Byomkesh Bakshi along with his friend Ajit Bandyopadhyay as he investigates the murder of a Pakistani spy named Heena Mallick. He begins the investigation at the request of a political leader, Mr. Santosh Samaddar, who was a close friend of Heena's parents and had given her shelter after her parents' death. Will Byomkesh succeed?