Baksho Rahashya.

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.