Byomkesh and Ajit are invited by the young Rai Bahadur Tridibendra Narayan Roy to recover the Seemanto Heera ("Frontier Diamond"), a family heirloom worth much more than its material value, from his uncle, the wealthy and brilliant Sir Digindra Narayan Roy (Utpal Dutt). Byomkesh must locate and recover the diamond while maintaining the utmost secrecy. He must also stay one step ahead of Digindra Narayan, who will do all he can to hold on to the trophy and conquer the intellect of Byomkesh.