Famous Bengali private investigator Pradosh C. Mitter (Feluda) is hired by wealthy businessman Dinnanath Lahiri. His suitcase has been accidentally exchanged with someone on the Kalka Mail. Inside it is a valuable manuscript written by Shambucharan Bose. The case takes Feluda, Topshe and Jatayu to Shimla, where they discover a priceless diamond in a film container, disguised as a betel - nut. 'The Three Musketeers' face great danger in Shimla, before Feluda eventually solves the case.