Rich businessman Dinnanath Lahiri hires renowned Bengali private eye Pradosh C. Mitter (Feluda). His bag has unintentionally been swapped with someone else on the Kalka Mail. It has a priceless manuscript that Shambucharan Bose wrote. The investigation leads Feluda, Topshe, and Jatayu to Shimla, where they find a valuable diamond hidden inside a film canister that was mistaken for a betel nut. Before Feluda finally cracks the case, "The Three Musketeers" are in grave peril in Shimla.
—MaganlalMeghraj
