Theory:

     When Dr Chanda came, Bepin Babu said, “I’m fine. It all came back as soon as I got off the train at Ranchi.”
 
     “A unique case,” said Dr Chanda. “I shall certainly write about it in a medical journal.”
 
     “The reason why I sent for you,” said Bepin Babu, “is that I have a pain in the hip from a fall I had in Ranchi. If you could prescribe a pain killer…”
Explanation:
 
Bepin was very clear after having read the letter. He understood that it was all a prank played by his friend Chunilal. In reality, there was nothing wrong with Bepin; and Bepin did not lose his memory as he assumed. Chunilal got Parimal Ghose acted to make Bepin believe he was mad. And, since Chunilal was Bepin's friend, there are possibilities that he might have known about his personal details like his reading habit, wife's death, and brother's illness. Bepin must have realised his mistake of not helping a friend during a crisis and must have understood the effect of his uncaring behaviour.
 
The doctor arrived at Bepin's location after some time had passed. Since Bepin was embarrassed to tell him the truth, he told the doctor he had regained his memory. Bepin informed the doctor that as soon as he stepped off the train at Ranchi, he regained his recollections of his previous Ranchi journey.
 
For the second time in his life, the doctor was taken aback when he visited Bepin. He considered publishing an article about Bepin's case in a medical publication.
 
The doctor's surprise.png
The doctor's surprise
 
Bepin proceeded to tell a lie to the doctor. He informed the doctor that he fell in Ranchi and had a hip injury, which is why he contacted him. Though Bepin had a hip injury, it was not the actual reason he called him. He then asked the doctor for a pain reliever. The story concludes with a humorous twist.
Reference:
National Council of Educational Research and Training (2008). Honeydew. Bepin Choudhury’s Lapse of Memory- Satyajit Ray (pp. 59-69). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.