The story revolves around the theft of a priceless heirloom, the Seemanto-heera or "Frontier Diamond", belonging to the heir of the Roy clan, one of the minor rulers of North Bengal.
Byomkesh and Ajit have been invited by the current heir, a young Rai Bahadur Tridibendra Narayan Roy to recover the diamond.
Tridibendran was the sole heir of his line according to the rules of succession where only the eldest son became ruler, but he did have an uncle, Sir Digindra Narayan Roy who was the younger brother of his father.
Consumed by greed, he did steal it while the diamond was on display in Calcutta and he replaced it with a fake that looked exactly the same to the untrained eye.
Tridibendra had sought the services of Byomkesh and Ajit to recover the diamond but also requested the utmost secrecy, since he didn't want the press to get wind of this loss.
Not one to lose heart, Byomkesh still boasted that he would find and recover the diamond by the end of the week to which the arrogant Sir Roy took immediate offence.
Searching for the diamond turned out to be an arduous task with the work been made even harder by the constant scorn of Sir Roy.