Bepin Babu’s car was safely parked in Bertram Street by the Lighthouse Cinema. He told the driver as he got into the car, “Just drive by the Ganga, will you, Sitaram.” Driving up the Strand Road, Bepin Babu regretted having paid so much attention to the intruder. He had never been to Ranchi — no question about it. It was inconceivable that he should forget such an incident which took place only six or seven years ago. He had an excellent memory. Unless — Bepin Babu’s head reeled.
     Was he losing his mind? But how could that be? He was working daily in his office. It was a big firm, and he was doing a responsible job. He wasn’t aware of anything ever going seriously wrong. Only today he spoke for half an hour at an important meeting. And yet...
Bepin Babu left the place after having spoken with Parimal. He then reached Lighthouse Cinema on Bertram Street, where Bepin Babu's car was safely parked. He asked his driver Sitaram to drive by Ganga. Bepin Babu felt bad for paying so much attention to Parimal's speech and wished he could have ignored it.

After all, Bepin was certain that he had never visited Ranchi. He did not have any second thought about the Ranchi trip, nor did he believe Ghose. It would be quite impossible for any normal man to forget a trip he had spent many days on; that too, the incident took place only some years before. Similarly, Bepin believed it was impossible when someone claimed to know Bepin and claimed that he had forgotten the entire trip event.

Above all, Bepin did not have memory issues, and he had good memory power. However, despite Bepin's confidence in his recall, he was disappointed by the bookshop incident.

There is no way Bepin Babu could be declared as a mad person because no such incident of memory loss occurred throughout his lifetime. As far as he knew, nothing had ever gone terribly wrong in his life. He also gave a half-hour speech at an important meeting at his office that day. Bepin Babu's remembering skills is appreciated by the author.

However, the word 'yet' in the paragraph brings out the contradiction. 'Yet' in here refers to Parimal Ghose's provided information on Bepin. During the conversation between Parimal and Bepin, Ghose provided some Bepin-related details to back up his claim. No one can dismiss them as the facts Ghose told about Bepin were true.
Meaning of difficult words:
IntruderSomeone who enters a place or situation where they are not wanted 
Inconceivable Impossible to think or imagine
ReelTo feel shocked or upset about something
ContradictoryBeing opposite to or not matching something
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.