Theory:

'Bepin Choudhury's Lapse of memory' is a humorous story about an aged introvert named Bepin. He preferred spending time alone with his books over spending time with people. One day, Bepin would encounter a stranger in a book shop. His name was Parimal Ghose. Parimal Ghose claimed to have known Bepin. Bepin was taken aback by Ghose's claim, and he tried his best to convince Parimal that he had mistaken Bepin for someone else. Parimal Ghose, on the other hand, was not convinced and listed Bepin's personal information such as his reading habits, wife's death, and brother's illness to back up his claim.
 
Parimal also informed Bepin that his friend Dinesh Mukherji had joined him on his Ranchi visit and that Bepin suffered a right knee injury after falling down in Hudroo. Even though Bepin did not trust Parimal Ghose's argument, he couldn't stop but wonder how Parimal knew so much about Bepin.
 
Bepin's peace of mind was disturbed as the bookstall event continued to bother him. He then turned his attention to his right knee, where he found an injury mark. Bepin had a good recall and had never had any problems with memory loss before. He felt confident about not having gone to Ranchi. He still wanted to clear things up, so he dialled his friend Dinesh's number to inquire about his Ranchi trip. Dinesh's response, however, was not in Bepin's favour.
 
Bepin then told his friend Chunilal about his dilemma, hoping for a favourable response. Chunilal, to his astonishment, also confirmed Bepin's journey to Ranchi.
 
Bepin confirmed his partial memory loss ailment after speaking with Ghose, Dinesh, and Chunilal and sought medical assistance. Following his doctor's advice, Bepin travelled to Ranchi to retrieve his memories. However, he returned to Calcutta with a heavy heart, having failed in his quest.
 
At last, a letter from Chunilal would aid in resolving Bepin's problem as Chunilal had plotted against Bepin with the help of Parimal Ghose and Dinesh. In reality, there was nothing wrong with  Bepin's memory. And, Chunilal played a prank on him as a sort of punishment for not helping him during a crisis.