 Feluda is approached by an established businessman, Mr. Dinanath Lahiri, claiming to have mistakenly swapped his suitcase in a train with one belonging to one of his co-passengers and asks Feluda to return it.This apparently simple problem takes Feluda, Topshe and Lalmohan babu to Shimla and into a realm of deceit and mystery involving a long-forgotten diamond and an old priceless manuscript titled "A Bengalee in Lamaland", written by Shambhu Charan Bose.The other major characters in the story are Dinanath Lahiri, a rich kindhearted businessman, his nephew, Prabeer Lahiri, a struggling actor obsessed with his own voice and Mr. Lahiri's co-passengers on the train