Cláudia has pointed out that the protagonist has a "wild itch" (his last name being Wilditch) to go exploring across the world, an urge implanted in him by Javitt, the emperor of the peculiar underground kingdom. The adult Wilditch has been diagnosed with cancer, an interior mystery - Cláudia suggests that in travelling back to the origins of his story, he is trying to solve a deeper mystery.
The story puts me in mind of these lines from Yeats from The Circus Animals' Desertion, which I have quoted before:
I must lie down where all the ladders start
In the foul rag and bone shop of the heart.
Further thoughts on the story will be in comments, since I know it's only some of my readers who are reading the Greene story. For next time, which will be May 30, let's read Gabriel Garcia Marquez's Story Collection The Incredible and Sad Tale of Innocent Eréndira and her Heartless Grandmother. I am going to keep Titus' suggestions in mind for the time after.