সত্যজিৎ রায়ের বক্স রহস্য

Feluda's new case revolves around an attaché case. A gentleman was returning from Delhi on the Kalka Mail. There were only four passengers in the first-class compartment, and somehow, his box got exchanged with someone else's. What was in the box? It contained the manuscript of a travelogue. Was that something valuable? Why did Feluda, after solving the mystery of the box, ask for the manuscript itself as his fee? To get the answer to that question, one needs to unravel the entire mystery.