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?.