Two briefcases exchanged on a train. A priceless manuscript. An ambush in a dark alley... A prosperous gentleman employs Feluda to recover his blue briefcase, which has got switched with another passenger's on the Kalka Mail. What starts out as an innocuous case soon becomes one of Feluda's most thrilling adventures. Feluda, Topshe and Jatayu set off for Simla in search of the briefcase, but there are unexpected twists and turns all the way. The hair-raising climax that unfolds on the snowy slopes of Simla throws light on a mystery that is devilishly complex.